大法师求解即可。

#include <bits/stdc++.h>
using namespace std;
const int N = 2005;
int f[N][N], n, m, ans;
int dfs(int A, int B) {
	if (A < 0 || B < 0 || A < B) return 0;
	if (f[A][B]) return f[A][B];
	return f[A][B] = dfs(A - 1, B) + dfs(A, B - 1);
}
int main() {
	cin >> n >> m, f[1][0] = f[1][1] = 1;
	cout << dfs(n, m) << '\n';
	return 0;
}