using System;

public class Hello
{
    public static void Main()
    {
        string[] line = Console.ReadLine().Trim().Split(' ');
        var a = long.Parse(line[0]);
        var b = long.Parse(line[1]);
        var c = long.Parse(line[2]);
        long ans;
        if (60 * a - b <= 0) ans = -1;
        else ans = (60 * a - b + 3600 * c - 1) / (60 * a - b);
        Console.WriteLine(ans);
    }
}