import java.util.Scanner;

public class Main {
    public static int n, a, b, x;
    
    public void getResult() {
        double[][] dp = new double[n + 1][x + 1];
        for(int j = a;j <= b;j++)
            dp[1][j] = 1.0 / (b - a + 1);
        for(int i = 2;i <= n;i++) {
            for(int k = a;k <= b;k++) {
                for(int j = a;j <= x;j++) {
                    if(j - k >= 0)
                        dp[i][j] += dp[i - 1][j - k] / (b - a + 1);
                }
            }
        }
        System.out.printf("%.4f",dp[n][x]);
    }
    
    public static void main(String[] args) {
        Main test = new Main();
        Scanner in = new Scanner(System.in);
        n = in.nextInt();
        a = in.nextInt();
        b = in.nextInt();
        x = in.nextInt();
        test.getResult();
    }
}