//Normal

#define _GLIBCXX_DEBUG
#define ll long long
#include <bits/stdc++.h>
using namespace std;
using Graph = vector<vector<int>>;

void print(auto a){
  cout << a;
}

void prints(auto a){
  cout << a << " ";
}

void prints(){
  cout << " ";
}

void printl(auto a){
  cout << a << endl;
}

void printl(){
  cout << endl;
}

void fix(int n){
  cout << fixed << setprecision(n);
}

int siz(string s){
  return (int)s.size();
}

int main(){
  ll A, B, C; cin >> A >> B >> C;

  if(A*60 <= B){
    printl(-1);
  }else{
    ll ng = -1, ok = 2e9;

    while(ng+1 < ok){
      ll mid = (ng+ok)/2;

      ll a = A*60*mid;
      ll b = B*mid + C*3600;
      b < a ? ok = mid : ng = mid;
    }

    printl(ok);
  }

  return 0;
}