#include <cstdio>
#include <cmath>
#define repeat_from(i,m,n) for (int i = (m); (i) < (n); ++(i))
int main() {
    long double x; scanf("%Lf", &x);
    long double y = 0;
    repeat_from (n,1,1000000) y += pow(x+n, -2);
    printf("%.12Lf\n", y);
    return 0;
}