Approximate

#include <iostream>
#include <cstdio>
#include <iomanip>
#include <vector>

using namespace std;

int main() {
	int n, q;
	cin >> n >> q;
	vector<int> s(n+1), cum(n+1), cum2(n+1);
	for(int i=1; i<=n; i++) {
		cin >> s[i];
		cum[i] = s[i];
		cum2[i] = s[i]*s[i];
		if(i) {
			cum[i] += cum[i-1];
			cum2[i] += cum2[i-1];
		}
		//cout << cum[i] << endl;
		//cout << cum2[i] << endl;
	}

	for(int i=0; i<q; i++) {
		int a, b;
		cin >> a >> b;
		double k = b-a+1;
		double s = (double)(cum[b]-cum[a-1]);
		double s2 = (double)(cum2[b]-cum2[a-1]);
		double c = s/k;
		double ans = (k*c*c - 2*s*c + s2)/k;
		printf("%.06f\n", ans);
		//cout << setprecision(6) << ans << endl;
	}
}

Test 1

Verdict: ACCEPTED