#include <bits/stdc++.h>
using namespace std;

#define all(x) begin(x), end(x)
#ifdef CKISEKI
#include <experimental/iterator>
#define safe cerr << __PRETTY_FUNCTION__ << " line " << __LINE__ << " safe\n"
#define debug(a...) debug_(#a, a)
#define orange(a...) orange_(#a, a)
void debug_(const char *s, auto ...a) {
  cerr << "\e[1;32m(" << s << ") = (";
  int f = 0;
  (..., (cerr << (f++ ? ", " : "") << a));
  cerr << ")\e[0m\n";
}
void orange_(const char *s, auto L, auto R) {
  cerr << "\e[1;33m[ " << s << " ] = [ ";
  using namespace experimental;
  copy(L, R, make_ostream_joiner(cerr, ", "));
  cerr << " ]\e[0m\n";
}
#else
#define safe ((void)0)
#define debug(...) safe
#define orange(...) safe
#endif

const int mod = 1000000007;
int modadd(int a, int b) {
  return a + b >= mod ? a + b - mod : a + b;
}
int modsub(int a, int b) {
  return a - b < 0 ? a - b + mod : a - b;
}
int modmul(int64_t a, int64_t b) {
  return static_cast<int>(a * b % mod);
}

int brute(int n, int k, string s) {
  int ans = 0;
  string p;
  for (int i = 0; i < n; i++)
    p += char(i + 'a');
  do {
    vector<string> sub;
    for (int i = 0; i < n; i++)
      for (int len = 1; i + len <= n; len++)
        sub.emplace_back(p.substr(i, len));
    sort(all(sub));
    if (sub[k - 1] == s)
      ++ans;
  } while (next_permutation(all(p)));
  return ans;
}

int main() {
  cin.tie(nullptr)->sync_with_stdio(false);
  int n, k;
  cin >> n >> k;
  string s;
  cin >> s;

  // int brute_ans = brute(n, k, s);
  // debug(brute_ans);

  const int ch = s[0] - 'a';

  bool used[26] = {};
  for (char c : s)
    used[c - 'a'] = true;

  int small = 0, big = 0;
  for (int i = 0; i < ch; i++)
    if (!used[i])
      small += 1;
  for (int i = ch + 1; i < n; i++)
    if (!used[i])
      big += 1;

  const int S = n * (n + 1) / 2;

  int ans = 0;

  for (int A = 0; A + s.size() <= n; A++) {
    auto dp = vector(small + 1, vector(S + 1, 0));
    int z = k - int(s.size());
    for (size_t j = 1; j < s.size(); j++) {
      if (s[j] < s[0]) {
        z -= A + (s.size() - j);
      }
    }

    if (z < 0) {
      continue;
    }

    dp[0][0] = 1;
    for (int w = 1; w <= n; w++) {
      // A + 1, A + 2, ... A + s.size()
      if (A + 1 <= w && w <= A + s.size()) continue;

      for (int c = small; c >= 1; c--) {
        for (int j = S; j >= w; j--) {
          dp[c][j] = modadd(dp[c][j], dp[c - 1][j - w]);
        }
      }
    }
    int cur = dp[small][z];
    debug(cur);

    ans = modadd(ans, cur);

  }

  for (int j = 1; j <= small; j++)
    ans = modmul(ans, j);
  for (int j = 1; j <= big; j++)
    ans = modmul(ans, j);

  debug(small, big);

  cout << ans << '\n';

  return 0;
}