using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    static int[][] NArr(long n) => Enumerable.Repeat(0, (int)n).Select(_ => NList).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var c = NList;
        var asec = c[0] * 60L;
        var bsec = (long)c[1];
        var csec = c[2] * 3600L;
        if (asec <= bsec) WriteLine(-1);
        else
        {
            // ax >= bx + c
            // x >= c / (a - b)
            WriteLine((csec + asec - bsec - 1) / (asec - bsec));
        }
    }
}