#include <bits/stdc++.h>
#define rep(i,n)for(int i=0;i<(n);i++)
#define MOD 1000000007
#define INF 0x3f3f3f3f
#define INFL 0x3f3f3f3f3f3f3f3f
#define EPS (1e-10)
using namespace std;
typedef long long ll;
typedef pair<int,int>P;

signed main(){
	ll a,b,c;cin>>a>>b>>c;
	a*=60;c*=3600;
	if(b>a){
		puts("-1");
	}
	else{
		cout<<(c+(a-b)-1)/(a-b)<<endl;
	}
}