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

int main() {
	double A,P,N;
	cin >> P >> N >> A;
	double res = (A-N)/(P-N)*1000;
	if (res >= 0 && P != N) cout <<fixed <<setprecision(1)<< res;
	else cout << -1;
	return 0;
}