POJ2478 Farey Sequence

 1 #include<cstdio>
 2 #include<iostream>
 3 using namespace std;
 4 int n,phi[1001000],tot,prime[500000];
 5 long long sum[1001000];
 6 bool check[1000100];
 7 void PHI(int n){
 8     phi[1] = 1;
 9     for(int i=2;i<=n;i++){
10         if(!check[i]){
11             prime[++tot] = i;
12             phi[i] = i - 1;
13         }
14         for(int j=1;j<=tot;j++){
15             if(prime[j]*i>n) break;
16             check[prime[j]*i] = true;
17             if(i%prime[j]==0){
18                 phi[i*prime[j]] = phi[i] * prime[j];
19                 break;
20             }else   phi[i*prime[j]] = phi[i] * (prime[j]-1);
21         }
22     }
23 }
24 
25 int main(){
26     PHI(1000000);
27     sum[2] = 1;
28     for(int i=3;i<=1000000;i++) sum[i]+=sum[i-1] + phi[i];
29     while(scanf("%d",&n)!=EOF && n) cout<<sum[n]<<endl;
30     return 0;
31 }