#include <bits/stdc++.h>
using namespace std;
const int MAX = 2010, MOD = 1e9 + 7;
int n, k;
char s[MAX];
long long dp[MAX][MAX], sum[MAX][MAX];
int main() {
  scanf("%d%d", &n, &k);
  scanf("%s", s + 1);
  memset(dp, 0, sizeof dp);
  memset(sum, 0, sizeof sum);
  dp[0][0] = 1;
  sum[0][0] = dp[0][0];
  for (int i = 1; i <= n; i++) {
    for (int j = 0; j <= k; j++) {
      dp[i][j] += sum[i - 1][j] * (s[i] - 'a');
      dp[i][j] %= MOD;
      for (int p = 0; i - p - 1 >= 0 && j - (p + 1) * (n - i + 1) >= 0; p++) {
        dp[i][j] += dp[i - p - 1][j - (p + 1) * (n - i + 1)] * ('z' - s[i]);
        dp[i][j] %= MOD;
      }
      sum[i][j] = sum[i - 1][j] + dp[i][j];
      sum[i][j] %= MOD;
    }
  }
  printf("%d\n", (int)sum[n][k]);
  return 0;
}
