diff --git a/include/int2048.h b/include/int2048.h
index 94164b9..509747f 100644
--- a/include/int2048.h
+++ b/include/int2048.h
@@ -48,7 +48,8 @@ root= 6
 
   __int128_t QuickPow(__int128_t v, long long q);
   void NTTTransform(__int128_t *, int, bool);
-  
+  friend int2048 GetInv(const int2048 &,int);
+
  public:
   int2048();
   int2048(long long);
@@ -114,7 +115,6 @@ root= 6
   friend bool operator<=(const int2048 &, const int2048 &);
   friend bool operator>=(const int2048 &, const int2048 &);
 };
-int2048 GetInv(const int2048 &,int);
 }  // namespace sjtu
 
 #endif
\ No newline at end of file
diff --git a/src/int2048.cpp b/src/int2048.cpp
index 0fb459f..8c5fdd4 100644
--- a/src/int2048.cpp
+++ b/src/int2048.cpp
@@ -562,7 +562,65 @@ void int2048::UnsignedMultiplyByInt(int v) {
     this->num_length--;
 }
 
-int2048 GetInv(const int2048 &, int) { ; }
+/**
+ * @brief Estimate the inverse of B, which is the result of [(base)^2n/B]
+ */
+int2048 GetInv(const int2048 &B, int n) {
+  const static int kPow10[9] = {1,      10,      100,      1000,     10000,
+                                100000, 1000000, 10000000, 100000000};
+  int total_blocks = (B.num_length + int2048::kNum - 1) / int2048::kNum;
+  if (n <= 2) {
+    long long b = B.val[total_blocks - 1] * (long long)int2048::kStoreBase +
+                  B.val[total_blocks - 2];
+    int2048 res;
+    res.ClaimMem((2 * n) * int2048::kNum + 1);
+    res.val[2 * n] = 1;
+    __uint128_t c = 0;
+    for (int i = 2 * n; i >= 0; i--) {
+      c = c * int2048::kStoreBase + res.val[i];
+      res.val[i] = c / b;
+      c %= b;
+      assert(res.val[i] < int2048::kStoreBase);
+    }
+    res.num_length = (2 * n) * int2048::kNum + 1;
+    while (res.num_length > 1 &&
+           res.val[(res.num_length - 1) / int2048::kNum] /
+                   kPow10[(res.num_length - 1) % int2048::kNum] ==
+               0)
+      res.num_length--;
+    return std::move(res);
+  }
+  int k = (n + 2) >> 1;
+  int2048 sub_soluton = std::move(GetInv(B, k));
+  int2048 sub_soluton_copy_1(sub_soluton);
+  int2048 sub_soluton_copy_2(sub_soluton);
+  sub_soluton_copy_1.UnsignedMultiplyByInt(2);
+  sub_soluton_copy_1.RightMoveBy((n - k) * int2048::kNum);
+  int2048 current_B;  // current_B is the highest n blocks of B
+  current_B.ClaimMem(n * int2048::kNum);
+  for (int i = n - 1; i >= 0; i--)
+    current_B.val[i] = B.val[i + total_blocks - n];
+  UnsignedMultiply(sub_soluton_copy_2, &current_B);
+  UnsignedMultiply(sub_soluton_copy_2, &sub_soluton);
+  sub_soluton_copy_2.RightMoveBy(2 * k * int2048::kNum);
+  UnsignedMinus(sub_soluton_copy_1, &sub_soluton_copy_2);
+  int2048 res = sub_soluton_copy_1;
+  int2048 remain;
+  remain.ClaimMem((2 * n) * int2048::kNum + 1);
+  remain.val[2 * n] = 1;
+  remain.num_length = (2 * n) * int2048::kNum + 1;
+  UnsignedMultiply(sub_soluton_copy_1, &current_B);
+  UnsignedMinus(remain, &sub_soluton_copy_1);
+  for (int i = 64; i > 0; i >>= 1) {
+    int2048 tmp_B(current_B);
+    tmp_B.UnsignedMultiplyByInt(i);
+    if(UnsignedCmp(remain, tmp_B) >= 0) {
+      res += i;
+      UnsignedMinus(remain, &tmp_B);
+    }
+  }
+  return std::move(res);
+}
 inline void UnsignedDivide(int2048 &A, const int2048 *pB) {
   int2048 B(*pB);
   int L1 = (A.num_length + int2048::kNum - 1) / int2048::kNum;
@@ -579,6 +637,7 @@ inline void UnsignedDivide(int2048 &A, const int2048 *pB) {
       c = c * int2048::kStoreBase + A.val[i];
       A.val[i] = c / b;
       c %= b;
+      assert(A.val[i] < int2048::kStoreBase);
     }
     A.num_length = L1 * int2048::kNum;
     const static int kPow10[9] = {1,      10,      100,      1000,     10000,
@@ -612,7 +671,13 @@ inline void UnsignedDivide(int2048 &A, const int2048 *pB) {
   UnsignedMinus(remain, &tmp_B);
   for (int i = 8; i > 0; i >>= 1) {
     tmp_B = B;
+    tmp_B.UnsignedMultiplyByInt(i);
+    if (UnsignedCmp(remain, tmp_B) >= 0) {
+      res_hat += i;
+      UnsignedMinus(remain, &tmp_B);
+    }
   }
+  A = std::move(res_hat);
 }
 int2048 &int2048::Divide(const int2048 &B) {
   if (this == &B) {
diff --git a/tester/cases/3.py b/tester/cases/3.py
index a1f1ffb..c93c5fd 100755
--- a/tester/cases/3.py
+++ b/tester/cases/3.py
@@ -36,7 +36,7 @@ opt_python=[]
 
 if True:
   for i in range(0,10):
-    val=randint(-10**15,10**15)
+    val=randint(-10**32,10**32)
     if i==0:
       val=randint(-10**100,10**100)
     opt_cpp.append("a_"+str(i)+"=int2048(\""+str(val)+"\");")