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