import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		while (true) {
			String ss = s.nextLine();
			if (ss.equals("0")) {
				s.close();
				return;
			}
			String[] sp = ss.split(" ");
			int c = Integer.valueOf(sp[0]);
			int n = Integer.valueOf(sp[1]);
			int m = Integer.valueOf(sp[2]);
			if (c == 0) {
				if (m != 0)
					System.out.println("0.000");
				else
					System.out.println("1.000");
				continue;
			}
			if (m > c || m > n) {
				System.out.println("0.000");
				continue;
			}
			if (m == 0 && n == 0) {
				System.out.println("1.000");
				continue;
			}

			float[][] dp = init(c, n, m);
			System.out.println(String.format("%.3f", dp[n][m]));
		}
	}

	static float[][] init(int c, int n, int m) {
		if (n > 1000)
			n = 1000 + n % 2;
		float[][] dp = new float[n + 1][c + 2];
		dp[0][0] = 1.0f;
		for (int i = 1; i <= n; ++i) {
			dp[i][0] = dp[i - 1][1] / (float) c;
			dp[i][c] = dp[i - 1][c - 1] / (float) c;
			for (int j = 1; j < c; ++j) {
				if ((j + i) % 2 == 1)
					continue;
				dp[i][j] = (dp[i - 1][j + 1] * (float) (j + 1) + dp[i - 1][j - 1]
						* (float) (c + 1 - j))
						/ (float) c;
			}
		}
		return dp;
	}
}