#include <iostream>
#include "mathlib.h"

using namespace std;
using namespace mathlib;

int main() {

		MathError err;
		double mean = 0.0; 
		double M2 = 0.0;
		int n = 0;
		double x;

		while(cin >> x) {
		
			n++;

			double delta = subtract(x, mean);
			mean = add(mean, divide(delta, (double)n, err));
			M2 = add(M2, multiply(delta, subtract(x, mean)));
		}

		if(n < 2) {
			cout << "Error: not enough data" << endl;
			return 1;
		}

		double variance = divide(M2, subtract((double)n, 1.0), err);
		double stddev = root(variance, 2, err);

		cout << stddev << endl;

		return 0;
}