#include <iostream>
#include <cmath>
using namespace std;
int main() {
	long long a,b,x,s,t,ans=-1;
	cin >> a >> b >> x;
	s=a+b;
	t=abs(a)+abs(b);
	(!x) && cout << 0 || 
	(x==a) && cout << abs(a) ||
	!((x - a) % (s)) && ((x - a) / (s)) * (t) + abs(a) >= 0 && cout << ((x - a) / (s)) * (t) + abs(a) ||
	!(x % (s)) && x / (s) * (t) >= 0 && cout << x / (s) * (t) || cout << -1;
	return 0;
}