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
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <functional>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>

#define FOR(i,a,b) for (int i = (a); i < (b); ++i)
#define REP(i,N) FOR(i,0,N)
#define FORD(i,b,a) for (int i = (int)(b)-1; i >= a; --i)

using namespace std;

typedef long long LL;
typedef pair<int, int> PII;
#define st first
#define nd second
#define pb push_back

const int MOD = 1000000007;

LL DP[3005][3005][2];

int main() {
  int N, M;
  scanf("%d%d", &N, &M);

  DP[0][0][1] = 1;
  REP(i,N) {
    REP(j,N+1) {
      (DP[i+1][j][1] += (DP[i][j][0] + DP[i][j][1]) * j) %= MOD;
      (DP[i+1][j][0] += DP[i][j][0] * (M-j)) %= MOD;
      (DP[i+1][j+1][0] += DP[i][j][1] * (M-j)) %= MOD;
    }
  }

  LL result = 0;
  REP(j,N+1) (result += DP[N][j][1]) %= MOD;
  printf("%lld\n", result);
}