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, Os;
Js = new TreeSet<Integer>();Is = new TreeSet<Integer>();Os = new TreeSet<Integer>();
st = new StringTokenizer(br.readLine());
String S = st.nextToken();
for (int i=0;i<N;i++) {
char c = S.charAt(i);
if (c=='J') Js.add(i);
else if (c=='I') Is.add(i);
else Os.add(i);
}
int ans = Integer.MAX_VALUE;
int iterations = 0;
if (Js.size()>0) {
int right = -1;
int left = Js.pollFirst();
int nLeft = next(Js, K-1, left);
if (nLeft>=0) {
int nnLeft = next(Os, K, nLeft);
if (nnLeft>0) {
right = next(Is, K, nnLeft);
}
}
if (right>0) ans = Math.min(ans, right-left+1-(3*K));
while (right>0 && Js.size()>0&&iterations<100000) {
left = Js.pollFirst();
right = -1;
int nL = next(Js, K-1, left);
if (nL>=0) {
int nnL = next(Os, K, nL);
if (nnL>0) {
right = next(Is, K, nnL);
}
}
if (right>0) ans = Math.min(ans, right-left+1-(3*K));
iterations++;
}
}
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;
}
}
