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

class Program {
    static void Main() {
        var hand = NextInt() * 60L;
        var runCost = NextInt();
        var initCost = NextInt() * 3600L;

        if (hand <= runCost) {
            WriteLine(-1);
            return;
        }
        WriteLine((initCost - 1) / (hand - runCost) + 1);
    }

    static int NextInt() {
        return int.Parse(NextString());
    }

    static string NextString() {
        var result = new List<char>();
        while (true) {
            int next = Read();
            if (next < 0)
                break;
            var nextChar = (char)next;
            if (!char.IsWhiteSpace(nextChar))
                result.Add(nextChar);
            else if (nextChar != '\r')
                break;
        }
        return string.Join("", result);
    }
}