1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
#include <algorithm>
#include <cstdio>

#define ll long long

using namespace std;

const ll B = 1000000007;

ll dp[3005][2][1505];

ll solve(int n, int m) {
  dp[1][0][1] = 1 % B;
  for(int i=1;i<n;i++) {
    int end = min(m, (i+1)/2);
    for(int j=1;j<=end;j++) {
      dp[i+1][1][j] += (j * dp[i][0][j]) % B;
      dp[i+1][1][j] %= B;
      dp[i+1][0][j] += (((m+B-j)%B) * dp[i][0][j]) % B;
      dp[i+1][0][j] %= B;
      dp[i+1][1][j] += (j * dp[i][1][j]) % B;
      dp[i+1][1][j] %= B;
      dp[i+1][0][j+1] += dp[i][1][j];
      dp[i+1][0][j+1] %= B;
    }
  }
  ll res = 0;
  ll d = m;
  int end = min(m, n/2);
  for(int i=1;i<=end;i++) {
    res += (dp[n][1][i] * d) % B;
    res %= B;
    d *= (m+B-i)%B;
    d %= B;
  }
  return res;
}

int main(int argc, char** argv) {
  int n, m;
  scanf("%d%d", &n, &m);
  ll res = solve(n, m);
  printf("%lld\n", res);
  return 0;
}