using namespace std;
#include<bits/stdc++.h>
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
typedef long long ll;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
#define all(v) v.begin(),v.end()
#define rep(i, star,fini) for (int i = (int)(star); i < (int)(fini); i++)
ll a,b,c;
int main() {
    cin >> a >> b >> c;
    a *= 60;
    c *= 3600;
    ll ans = c/(a-b);
    if(a<b){cout<<-1<<endl;return 0;}
    if(ans*a>ans*b+c)
    {
        ans += 1;
    }
    cout<<ans+1<<endl;
}