#include <bits/stdc++.h>
using namespace std;
long long MOD = 1000000007;
long long dp[2001][2001];
long long dp1[2001][2001];
int main() {
  int N, K;
  string S;
  cin >> N >> K >> S;
  memset(dp, 0, sizeof(dp));
  memset(dp1, 0, sizeof(dp1));
  dp[0][0] = dp1[0][0] = 1;
  for (int n = 1; n <= N; ++n) {
    for (int k = 0; k <= K; ++k) {
      dp[n][k] = dp1[n - 1][k] * (long long)(S[n - 1] - 'a') % MOD;
    }
    for (int i = 0; i < n; ++i) {
      int delta = (n - i) * (N - n + 1);
      for (int k = 0; k + delta <= K; ++k) {
        dp[n][k + delta] += dp[i][k] * (long long)('z' - S[n - 1]) % MOD;
        dp[n][k + delta] %= MOD;
      }
    }
    for (int k = 0; k <= K; ++k) {
      dp1[n][k] = (dp1[n - 1][k] + dp[n][k]) % MOD;
    }
  }
  cout << dp1[N][K] << endl;
  return 0;
}
