#pragma GCC target("avx2")
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>

int ri() {
	int n;
	scanf("%d", &n);
	return n;
}
#define LOOP 40003
int main() {
	int n = ri();
	assert(n > 5);
	long double ans = 0;
	std::vector<long double> sins(LOOP + 1);
	std::vector<long double> coss(LOOP + 1);
	for (int i = 1; i <= LOOP; i++) sins[i] = sin((long double)i), coss[i] = cos((long double)i);
	for (int i = 0; i < n; i++) {
		long double x, a;
		scanf("%Lf%Lf", &x, &a);
		long double r0 = sin(x) * x;
		long double r1 = cos(x);
		long double r2 = x * x;
		long double res = 0;
		for (int j = LOOP; j; j--) res += (r0 * coss[j] - r1 * j * sins[j]) / (r2 - j * j);
		res *= 2;
		res += sin(x) / x;
		ans += res * a;
	}
	printf("%.12Lf\n", ans);
	return 0;
}