#include <iostream>
#define llint long long
#define mod 1000000007

using namespace std;

llint n, k;
llint dp[305][305];
llint dp2[305][305];

int main(void)
{
  cin >> n >> k;
  dp[0][0] = 1;
  for(int i = 0; i < k; i++){
    for(int j = 0; j <= k+1; j++){
      dp[i+1][j] += dp[i][j] * (n-1) % mod;
      dp[i+1][j] %= mod;
      if(j+1 <= k+1) dp[i+1][j+1] += dp[i][j];
      if(j+1 <= k+1) dp[i+1][j+1] %= mod;
    }
  }
  dp2[0][0] = 1;
  for(int i = 0; i < k; i++){
    for(int j = 0; j <= k+1; j++){
      if(j==0) dp2[i+1][j] += dp2[i][j] * (n-2) % mod;
      else dp2[i+1][j] += dp2[i][j] * (n-1) % mod;
      dp2[i+1][j] %= mod;
      if(j+1 <= k+1) dp2[i+1][j+1] += dp2[i][j];
      if(j+1 <= k+1) dp2[i+1][j+1] %= mod;
    }
  }
  /*for(int i = 0; i <= k; i++){
    for(int j = 0; j <= k+1; j++){
    cout << dp[i][j] << " ";
    }
    cout << endl;
  }*/

  llint ans = 0;
  for(int i = 1; i <= n; i++){
    llint cnt = 0;
    for(int j = 1; j <= min(k, n-1); j++){
      //cout << dp[k][j] << endl;
      cnt += (dp[k][j] - dp2[k][j+1] + mod) % mod, cnt %= mod;
    }
    ans += cnt * i % mod;
    ans %= mod;
  }
  cout << ans << endl;

  return 0;
}