#include<bits/stdc++.h>

using namespace std;
using ll=long long;
#define rep2(i, a, n) for(ll i = (a); i < (n); i++)
#define rep(i, n) rep2(i,0,n)

int main(){
  cin.tie(nullptr);ios_base::sync_with_stdio(false);
  ll a,b,c;
  cin>>a>>b>>c;
  int hoge=-1;
  rep(i,1e8){
    if(i*(a*60-b)>=c*3600){
      hoge=i;
      break;
    }
  }
  cout<<hoge<<endl;
}