import java.util.*;
import java.io.*;
public class ho_t2 {
public static void main(String[] args) throws IOException {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
PrintWriter pw = new PrintWriter(System.out);
StringTokenizer st = new StringTokenizer(br.readLine());
int N = Integer.parseInt(st.nextToken());
int K = Integer.parseInt(st.nextToken());
TreeSet<Integer> Js, Is;
Js = new TreeSet<Integer>();Is = new TreeSet<Integer>();
int [] O = new int [N];
int [] J = new int [N];
st = new StringTokenizer(br.readLine());
String S = st.nextToken();
O[0] = 0;J[0] = 0;
for (int i=0;i<N;i++) {
char c = S.charAt(i);
if (i!=0) {
O[i]=O[i-1];
J[i]=J[i-1];
}
if (c=='J') {
Js.add(i);
J[i]++;
}
else if (c=='I') Is.add(i);
else O[i]++;
}
int ans = Integer.MAX_VALUE;
int left = Js.pollFirst();
int right = next(Is, K, left);
while (right>0) {
int numOs = O[right] - O[left];
int numJs = J[right] - J[left]+1;;
if (numOs<K || numJs<K) {
right = next(Is, K, right);
} else {
ans = Math.min(ans, right-left+1-(3*K));
left = Js.pollFirst();
right = next(Is, K, left);
}
}
if (ans==Integer.MAX_VALUE) ans = -1;
pw.println(ans);
pw.close();
br.close();
}
public static int next(TreeSet<Integer> s, int n, int curr) {
int counter = n;
int toReturn = curr;
while (counter>0 && s.higher(toReturn)!=null) {
toReturn = s.higher(toReturn);
counter--;
}
if (counter>0) return -1;
return toReturn;
}
}
