#include <stdio.h>
#include <windows.h>

int main(){
    int n, i = 0;
    float sum = 0.0;
    scanf("%d", %n);
    for (i=0; i<n; i++){
        sum += 3.0/(n*n+2*n);
    }
    printf("%f", &sum);
    system("pause");
    return 0;
}