#include <iostream>
#include <string>
const int INF = 1e+9;
const int NMAX = 1e+5;
using namespace std;

int k, n;
char s[NMAX];
int dp[26][NMAX];
int curr_maxx[NMAX];
int last[26];
int main () {
  freopen ("input.txt", "r", stdin);
  freopen ("output.txt", "w", stdout);
  scanf("%d%d", &n, &k);
  scanf("%s", s);
  dp[s[0] - 'A'][0] = 0;
  for (int j = 0; j < 26; j++) last[j] = INF;
  last[s[0] - 'A'] = 0;
  for (int i = 1; i < n; i++) {
    int minn = INF;
    for (int j = 0; j < 26; j++) {
      dp[i][j] = INF;
      if (i - last[j] > k || last[j] == INF)
        continue;
      dp[i][j] = curr_maxx[last[j]] + (j != int(s[i] - 'A'));
      minn = min(dp[i][j], minn);
    }
    curr_maxx[i] = minn;
    last[s[i] - 'A'] = i;
  }
  printf("%d", curr_maxx[n - 1]);
  return 0;
}
/*
10 2
ABABBCACBC
*/