不是很会，记录一下

最后的下边换成形同下标
1 * d1 + 2 * d2 + 3 * d3 + … +(n - 1)dn-1 = s(mod(n))

import java.util.*;

public class Main {
    static final int N = 1010, MOD = 100000007;
    static int[][] f = new int[N][N];

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt(), s = sc.nextInt(), a = sc.nextInt(), b = sc.nextInt();

        f[0][0] = 1;
        for (int i = 1; i <= n; i++)
            for (int j = 0; j < n; j++)
                f[i][j] = (f[i - 1][get_mod(j - i * a, n)] + f[i - 1][get_mod(j + b * i, n)]) % MOD;

        System.out.println(f[n - 1][get_mod(s, n)]);
    }

    static int get_mod(int a, int m) {
        return (a % m + m) % m;
    }
}