#include <iostream>
#include <iomanip>
using namespace std;


int main() {
	int n;
	cin >> n;
	
	double res;
	
	for (int i = 0; i < n; i++) {
		double v; bool mode;
		
		cin >> mode;
		cin >> v;
		
		if (mode) res += 1 / v;
		else res -= 1 / v;
	}
	
	if (res > 0) {
		cout << setprecision(4) << 1 / res;
	} else {
		cout << -1;
	}
	
	
	return 0;
}