1 #include<cstdio> 2 #include<algorithm> 3 #include<iostream> 4 #include<cstring> 5 #include<queue> 6 #include<cmath> 7 using namespace std; 8 const int N = 1000010; 9 const double pi = acos(-1.0);10 char a[N];11 double sum[N];12 13 int main(){14     int i, l, x;15     for(int i = 1; i < N; ++i)16         sum[i] = sum[i - 1] + 1.0/i/i;17     while(~scanf("%s",a)){18         l = strlen(a);19         if(l >= 7)printf("%.5lf\n",pi*pi/6);20         else{21             for(x = i = 0; i < l; ++i)22                 x = 10 * x + a[i] - ‘0‘;23             printf("%.5lf\n", sum[x]);24         }25     }26     return 0;27 }