#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;
}

int main() {
	int n = ri();
	double ans = 0;
	for (int i = 0; i < n; i++) {
		double x, a;
		scanf("%lf%lf", &x, &a);
		double res = sin(x) / x;
		for (int j = 1; j <= 20000; j++) res += sin(x + j) / (x + j) + sin(x - j) / (x - j);
		ans += res * a;
	}
	printf("%.12f\n", ans);
	return 0;
}