这其实是一道DP题

#include <iostream>
#include <vector>

using namespace std;
int m, n;

int main(void) {
  scanf("%d %d", &m, &n);
  vector<vector<int>> dp(m + 1, vector<int>(n + 1, 1));
  int ans = 0;
  for (int y = 1; y <= m; ++y)
    for (int x = 1; x <= n; ++x)
      dp[y][x] = dp[y][x - 1] + dp[y - 1][x];

  printf("%d\n", dp[m][n]);
  return 0;
}