#include <bits/stdc++.h>
using namespace std;
const int Mod = 1e9 + 7;
int add(int a, int b) { return a + b >= Mod ? a + b - Mod : a + b; }
int dec(int a, int b) { return a - b < 0 ? a - b + Mod : a - b; }
int mul(int a, int b) { return 1ll * a * b % Mod; }
void Add(int &a, int b) { a = add(a, b); }
void Dec(int &a, int b) { a = dec(a, b); }
void Mul(int &a, int b) { a = mul(a, b); }
int ksm(int a, int b) {
  int ans = 1;
  for (; b; b >>= 1, Mul(a, a))
    if (b & 1) Mul(ans, a);
  return ans;
}
const int N = 1e6 + 50;
int n, m, fc[N], ifc[N];
void init(int n) {
  fc[0] = ifc[0] = 1;
  for (int i = 1; i <= n; i++) fc[i] = mul(fc[i - 1], i);
  ifc[n] = ksm(fc[n], Mod - 2);
  for (int i = n - 1; i; i--) ifc[i] = mul(ifc[i + 1], i + 1);
}
int C(int n, int m) {
  if (n < 0 || m < 0 || n < m) return 0;
  return mul(fc[n], mul(ifc[n - m], ifc[m]));
}
int main() {
  cin >> n >> m;
  init(max(n, m));
  static int pm[N], pn[N];
  pm[0] = pn[0] = 1;
  for (int i = 1; i <= n; i++)
    pm[i] = mul(pm[i - 1], m), pn[i] = mul(pn[i - 1], n);
  int ans = 0;
  for (int i = 2; i <= n; i++) {
    if (i - 2 > m - 1) break;
    int c = mul(C(m - 1, i - 2), C(n - 2, i - 2));
    Mul(c, pm[n - i]), Mul(c, fc[i - 2]);
    Mul(c, i < n ? mul(i, pn[n - i - 1]) : 1);
    Add(ans, c);
  }
  cout << ans;
  return 0;
}
