RT，求助。

// Author: Qing_fy(QingFy)

#include <bits/stdc++.h>
using namespace std;

struct io_Faster {
    io_Faster() {
		ios::sync_with_stdio(false);
	    cin.tie(NULL); cout.tie(NULL);
	}
} IO_Faster;

typedef long long ll;

int up[25], down[25], people[25];
// up(i): 第i站上车人数
// down(i): 第i站下车人数
// people(i): 第i站开出后车上人数 

int main() {                                  
	int a, n, m, x;
	cin >> a >> n >> m >> x;
	up[1] = a;
	down[1] = 0;
	people[1] = a;
	if (n >= 2) {
		people[2] = a;
		for (up[2] = down[2] = 0; up[2] <= a; up[2]++, down[2]++) { // 枚举第2站的上下车人数 
			for (int i = 3; i < n; i++) {
				up[i] = up[i - 2] + up[i - 1];
				down[i] = up[i - 1];
				people[i] = people[i - 1] + up[i] - down[i];
			}
			people[n] = 0;
			if (people[n - 1] == m) break;
		}
	}
	cout << people[x] << endl;
	return 0;
}