#include<iostream>
#include<algorithm>
#include<cmath>
using namespace std;
int main() {
  long long a,b,c,ans=0,A=0,B;
  cin >> a >> b >> c;
  B=c*3600;
  if(60*a<=b) {
      cout << -1 << endl;
  }
  else {
      while(A<B) {
          A+=60*a,B+=b;
          ans++;
      }
  }
  cout << ans+1<< endl;
}