using System;
using System.Globalization;
namespace FilmRating
{
    class Program
    {
        static int calculateTask(double currentScore, double needScore, double countScores)
        {
            if (currentScore <= needScore)
                return 0;
            if (currentScore < 10 && needScore > 1) { }
                currentScore += 0.049999999999999999999999999999;
            needScore += 0.049999999999999999999999999999;
            var sumScore = currentScore * countScores;
            var k = (sumScore - needScore * countScores) / (needScore - 1);
            return (int) Math.Round(k); //WA 8
        }
        static void Main()
        {
            var k = Console.ReadLine().Split();
            var x = double.Parse(k[0], CultureInfo.CurrentCulture);
            var y = double.Parse(k[1], CultureInfo.CurrentCulture);
            var n = Int32.Parse(k[2]);
            Console.WriteLine(calculateTask(x, y, n).ToString());
        }
    }
}