#include <bits/stdc++.h>
using namespace std;
inline void setmin(int &x, int y) {
  if (y < x) x = y;
}
inline void setmax(int &x, int y) {
  if (y > x) x = y;
}
inline void setmin(long long &x, long long y) {
  if (y < x) x = y;
}
inline void setmax(long long &x, long long y) {
  if (y > x) x = y;
}
const int N = 1000000;
const int inf = (int)1e9 + 1;
const long long big = (long long)1e18 + 1;
const int P = 239;
const int P1 = 31;
const int P2 = 57;
const int MOD = (int)1e9 + 7;
const int MOD1 = (int)1e9 + 9;
const int MOD2 = 998244353;
const double eps = 1e-9;
const double pi = atan2(0, -1);
const int ABC = 26;
long long bin_pow(long long a, long long p) {
  long long res = 1;
  while (p) {
    if (p & 1) {
      res = res * a % MOD;
    }
    a = a * a % MOD;
    p >>= 1;
  }
  return res;
}
long long rev(long long x) { return bin_pow(x, MOD - 2); }
long long fact[N + 1], rfact[N + 1];
void init() {
  fact[0] = 1;
  for (int i = 1; i < N + 1; i++) {
    fact[i] = fact[i - 1] * i % MOD;
  }
  rfact[N] = rev(fact[N]);
  for (int i = N - 1; i > -1; i--) {
    rfact[i] = rfact[i + 1] * (i + 1) % MOD;
  }
}
long long A(int n, int k) {
  if (k < 0 || k > n) {
    return 0;
  }
  return fact[n] * rfact[n - k] % MOD;
}
long long C(int n, int k) {
  if (k < 0 || k > n) {
    return 0;
  }
  return fact[n] * rfact[k] % MOD * rfact[n - k] % MOD;
}
long long get(int n, int k) {
  if (k <= 0 || k > n) {
    return 0;
  }
  if (k == n) {
    return 1;
  }
  return k * bin_pow(n, n - k - 1) % MOD;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.precision(20);
  cout << fixed;
  init();
  int n, m, a, b;
  cin >> n >> m >> a >> b;
  long long ans = 0;
  for (int len = 1; len < n; len++) {
    if (len > m) {
      continue;
    }
    long long cur = C((m - len) + len - 1, len - 1);
    cur = cur * A(n - 2, len - 1) % MOD;
    if (len + 1 < n) {
      cur = cur * get(n, len + 1) % MOD;
    }
    cur = cur * bin_pow(m, (n - 1) - len) % MOD;
    ans = (ans + cur) % MOD;
  }
  cout << ans << "\n";
  return 0;
}
