delete pics to save space

8.2818.cpp

@@ -0,0 +1,30 @@
+#include<cstdio>
+#include<algorithm>
+using namespace std;
+typedef long long LL;
+const int maxn=1e7+5;
+int n,p[maxn],v[maxn],phi[maxn],cnt;
+LL s[maxn];
+int main()
+{
+#ifdef local
+	freopen("pro.in","r",stdin);
+#endif
+	scanf("%d",&n);
+	for(int i=1;i<=n;i++) v[i]=i;
+	phi[1]=1;
+	for(int i=2;i<=n;i++)
+	{
+		if(v[i]==i) { p[cnt++]=i; phi[i]=i-1; }
+		for(int j=0;j<cnt&&(LL)p[j]*i<=n&&p[j]<=v[i];j++)
+		{
+			v[p[j]*i]=p[j];
+			phi[p[j]*i]=phi[i]*(p[j]<v[i]?p[j]-1:p[j]);
+		}
+	}
+	for(int i=1;i<=n;i++) s[i]=s[i-1]+phi[i];
+	long long res=0;
+	for(int i=0;i<cnt;i++) res+=2*s[n/p[i]]-1;
+	printf("%lld\n",res);
+	return 0;
+}