#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;
  cout<<(60*a<=b?-1:(long long)ceil((double)3600*c/(60*a-b)))<<endl;
}