[POJ2478]Farey Sequence

 1 #include <iostream>
 2 #include <cstdio>
 3 #include <cstring>
 4 #include <algorithm>
 5 #include <string>
 6 #include <map>
 7 #include <set>
 8 #include <cmath>
 9 #include <vector>
10 #include <deque>
11 #include <queue>
12 
13 using namespace std;
14 
15 typedef long long LL;
16 const int maxn = 1e6+1;
17 LL phi[maxn];
18 
19 void geteular() {
20     memset(phi, 0, sizeof(phi));
21     phi[1] = 1;
22     for(int i = 2; i < maxn; i++) {
23         if(!phi[i]) {
24             for(int j = i; j < maxn; j+=i) {
25                 if(!phi[j]) {
26                     phi[j] = j;
27                 }
28                 phi[j] = phi[j] / i * (i - 1);
29             }
30         }
31     }
32 }
33 int main() {
34     geteular();
35     int n;
36     while(~scanf("%d", &n) && n) {
37         LL sum = 0;
38         for(int i = 2; i <= n; i++) {
39             sum += phi[i];
40         }
41         printf("%I64d\n", sum);
42     }
43 }