#include <bits/stdc++.h>
using longlong=long long;

int main(){
    std::cin.tie(nullptr);std::cin.sync_with_stdio(false);
    longlong a,b,c;std::cin>>a>>b>>c;
    a*=60,c*=3600;
    std::cout<<(a<=b?-1:(c+a-b-1)/(a-b))<<std::endl;
}