Approximate

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

int main()
{
	int n, q;
	std::cin >> n >> q;
	std::vector<int> c;
	c.reserve(n);
	for (int i = 0; i < n; ++i) {
		int x;
		std::cin >> x;
		c.push_back(x);
	}

	for (int i = 0; i < q; ++i) {
		int a, b;
		std::cin >> a >> b;

		double d = 0.0;
		for (int j = (a - 1); j < b; ++j) {
			d += c[j];
		}
		d /= (b - a + 1);

		double err = 0.0;
		for (int j = (a - 1); j < b; ++j) {
			double diff = d - c[j];
			err += diff * diff;
		}
		err /= (b - a + 1);

		printf("%.6f\n", err);
	}
}

Test 1

Verdict: TIME LIMIT EXCEEDED