P2.13 P65171 Control C202B

#include <iostream>
using namespace std;


// [1/(1-n)] * x2   -   [1/(n(n-1))] * (x^2)



int main () {

    cout.setf(ios::fixed);
    cout.precision(2);


    double n, x, x1, x2;
    cin >> n;
    x=0;
    x1=0;
    x2=0;
    while (cin >> x) {
        x1 += x;
        x2 += (x*x);
    }
    x1 = (x1*x1)/(n*(n-1));
    x2 /= (n-1);
    cout << (x2-x1) << endl;
}