#include <bits/stdc++.h>
long long D[26], S, Mod = 1000000007;
char p[1010000];
int pp[26], n, m, K;
int main() {
  int x, i, j, MM;
  scanf("%d%d", &n, &K);
  scanf("%s", p + 1);
  for (i = 1; p[i]; i++) {
    x = p[i] - 'a';
    S = 1;
    for (j = 0; j < K; j++) {
      if (j != x) S += D[j];
    }
    pp[x] = i;
    D[x] = (D[x] + S) % Mod;
  }
  m = i - 1;
  for (i = m + 1; i <= m + n; i++) {
    MM = m + n + 1;
    for (j = 0; j < K; j++) {
      if (MM > pp[j]) {
        MM = pp[j];
        x = j;
      }
    }
    S = 1;
    for (j = 0; j < K; j++) {
      if (j != x) S += D[j];
    }
    pp[x] = i;
    D[x] = (D[x] + S) % Mod;
  }
  S = 1;
  for (i = 0; i < K; i++) S = (S + D[i]) % Mod;
  printf("%lld\n", S);
}
