#include <bits/stdc++.h>
using namespace std;
long long mod = (1000000007LL);
inline long long Mod(long long a, long long b) { return (a % b); }
inline long long poww(long long a, long long b) {
  long long res = 1;
  while (b > 0) {
    if (b & 1) res = (res * a) % mod;
    a = (a * a) % mod;
    b >>= 1;
  }
  return res;
}
long long gcd(long long a, long long b) {
  while (b) {
    a %= b, swap(a, b);
  }
  return a;
}
void read(vector<long long> &w, long long n) {
  w.resize(n);
  for (long long i = 0; i < n; i++) cin >> w[i];
}
void print(vector<long long> &w) {
  for (long long i = 0; i < (long long)(w).size(); i++) {
    if (i == (long long)(w).size() - 1)
      cout << w[i] << "\n";
    else
      cout << w[i] << " ";
  }
}
long long prodmod(vector<long long> w);
void crivo(vector<long long> &w, long long TAM);
void inverso(vector<long long> &inv, long long TAM);
void fat_inverso(vector<long long> &inv, long long TAM);
long long choose(long long a, long long b);
void get_fat();
long long fat[2000000];
long long n, m, a, b;
int32_t main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cin >> n >> m >> a >> b;
  long long ans = 0;
  fat[0] = 1;
  for (long long i = 1; i < 2000000; i++) fat[i] = (i * fat[i - 1]) % mod;
  for (long long k = 2; k <= n; k++) {
    long long A = (choose(n - 2, k - 2) * fat[k - 2]) % mod;
    long long B = (choose(m - 1, k - 2) * poww(m, n - k)) % mod;
    long long C =
        (n - k - 1 >= 0 ? (k * poww(n, n - k - 1)) : (k * poww(n, mod - 2))) %
        mod;
    A = prodmod({A, B, C});
    ans += A;
    ans %= mod;
  }
  cout << ans << "\n";
}
long long prodmod(vector<long long> w) {
  long long curr = 1;
  for (long long i = 0; i < (long long)(w).size(); i++) {
    if (w[i] >= mod) w[i] %= mod;
    curr = (curr * w[i]) % mod;
    if (curr < 0) curr += mod;
  }
  return curr;
}
void get_fat() {
  if (fat[0] != 0) return;
  fat[0] = 1;
  for (long long i = 1; i < 2000000; i++) fat[i] = (i * fat[i - 1]) % mod;
}
long long choose(long long a, long long b) {
  get_fat();
  if (b > a) return 0;
  if (!b) return 1;
  long long A = fat[a];
  long long B = (poww(fat[b], mod - 2) * poww(fat[a - b], mod - 2)) % mod;
  A = (A * B) % mod;
  return A;
}
void inverso(vector<long long> &inv, long long TAM) {
  inv.resize(TAM);
  inv[1] = 1;
  for (long long i = 2; i < TAM; ++i) {
    inv[i] = (mod - (mod / i) * inv[mod % i] % mod) % mod;
  }
}
void fat_inverso(vector<long long> &inv, long long TAM) {
  inverso(inv, TAM);
  for (long long i = 2; i < TAM; i++) inv[i] = (inv[i] * inv[i - 1]) % mod;
}
void crivo(vector<long long> &w, long long TAM) {
  w.resize(TAM + 1);
  w[0] = 1;
  for (long long i = 2; i < TAM; i++)
    if (!w[i])
      for (long long j = 2 * i; j < TAM; j += i) w[j] = 1;
}
