#include <bits/stdc++.h>
#define rep(i,n) for (int i=0; i<(n); ++i)
using namespace std;
using ll=long long;
int main(){
    int a,b,c;
    cin>>a>>b>>c;
    a*=60;
    c*=3600;
    if(a<=b){
        cout<<-1<<endl;
        return 0;
    }
    int ans=0;
    int x1=0,x2=c;
    while(x1<x2){
        x1+=a;
        x2+=b;
        ans++;
    }
    cout<<ans<<endl;
}