算法提高 概率计算

import java.io.BufferedInputStream;
import java.math.BigDecimal;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Scanner in = new Scanner(new BufferedInputStream(System.in));
        int num = in.nextInt();
        int start = in.nextInt();
        int end = in.nextInt();
        int sum = in.nextInt();
        double[][] dp = new double[num+1][sum+1];
        dp[0][0]= 1;

        for ( int i = 1 ; i <= num ; i++){
            for ( int j = 0; j <= sum ; j++){
                for ( int k = 0 ; k <= sum ; k++){
                    if ( j-k >= start && j-k <= end){
                        dp[i][j] += dp[i-1][k]/(1.0*(end-start+1));
                    }
                }
            }
        }
        System.out.println(new BigDecimal(dp[num][sum]).divide(BigDecimal.ONE, 4,BigDecimal.ROUND_HALF_UP));
        in.close();
    }

}