#include <bits/stdc++.h>
using namespace std;
int n, m, k;
char s[1000010];
const int MOD = 1000000007;
int lst[26];
set<pair<int, char> > pos;
int dp[26], sum = 0;
int main() {
  scanf("%d %d\n%s", &n, &k, s);
  m = strlen(s);
  for (int i = 0; i < k; ++i) {
    lst[i] = -1;
    pos.insert(make_pair(-1, 'a' + i));
  }
  for (int i = 0; i < m; ++i) {
    sum = (sum - dp[s[i] - 'a'] + MOD) % MOD;
    pos.erase(make_pair(lst[s[i] - 'a'], s[i]));
    dp[s[i] - 'a'] = (1 + dp[s[i] - 'a'] + sum) % MOD;
    sum = (sum + dp[s[i] - 'a']) % MOD;
    lst[s[i] - 'a'] = i;
    pos.insert(make_pair(lst[s[i] - 'a'], s[i]));
  }
  for (int i = m; i < m + n; ++i) {
    char id = pos.begin()->second;
    sum = (sum - dp[id - 'a'] + MOD) % MOD;
    pos.erase(make_pair(lst[id - 'a'], id));
    dp[id - 'a'] = (1 + dp[id - 'a'] + sum) % MOD;
    sum = (sum + dp[id - 'a']) % MOD;
    lst[id - 'a'] = i;
    pos.insert(make_pair(lst[id - 'a'], id));
  }
  printf("%d\n", (sum + 1) % MOD);
  return 0;
}
