#include <bits/stdc++.h>
#include <unordered_set>
using namespace std;
#define ll long long
#define rep(i,n) for(int (i)=0;(i)<(n);(i)++)

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout<<setprecision(std::numeric_limits<float>::max_digits10);
  ll a,b,c;
  cin>>a>>b>>c;
  if(60*a-b<=0) {
    cout << "-1" << endl;
  } else {
    cout << (3600*c+60*a-b-1)/(60*a-b) << endl;
  }
  return 0;
}