#include <bits/stdc++.h>
using namespace std;
int N, K;
int lftmst[4][200005];
string s;
const string joi = "JOI";
int main(){
cin.sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
cin >> N >> K >> s;
for(int i = 1; i<=N; i++){
lftmst[0][i] = i;
}
for(int t = 1; t<=3; t++){
int cnt = 0;
for(int l = 1, r = 1; r<=N; r++){
if(s[r-1] == joi[t-1]){
cnt++;
}
while(cnt > K){
cnt -= (s[l-1] == joi[t-1]);
l++;
}
while(cnt == K && l < r && s[l-1] != joi[t-1]){
l++;
}
//cout << l << " " << r << "\n";
if(cnt == K){
lftmst[t][r] = lftmst[t-1][l];
}
//cout << lftmst[t][r] << " " << r << "\n";
}
//cout << "\n";
}
int rng = N+2;
for(int i = 1; i<=N; i++){
if(lftmst[3][i]) {
rng = min(rng, i-lftmst[3][i]+1);
}
}
if(rng == N+2){
rng = -1;
}
else{
rng -= 3*K;
}
cout << rng << "\n";
}
