#include<iostream>
#include<string>
#include<vector>
#include<algorithm>
#define ll long long
using namespace std;
int main(){
ios::sync_with_stdio(false);
cin.tie(0);
string s;
int n,k,jcnt=0,ocnt=0,icnt=0;
cin>>n>>k;
cin>>s;
vector<int> jj,oo,ii;
for(int i=0;i<n;i++){
if(s[i]=='J') jj.push_back(i);
else if(s[i]=='O') oo.push_back(i);
else ii.push_back(i);
}
int ans=1e9;
jcnt=jj.size();
ocnt=oo.size();
icnt=ii.size();
for(int i=0;i+k-1<jcnt;i++){
int ooo=lower_bound(oo.begin(),oo.end(),jj[i+k-1])-oo.begin();
if(ooo+k-1>=ocnt) continue;
int iii=lower_bound(ii.begin(),ii.end(),oo[ooo+k-1])-ii.begin();
if(iii+k-1>=icnt) continue;
else ans=min(ans,ii[iii+k-1]-jj[i]+1-k*3);
}
if(ans==1e9) cout<<-1;
else cout<<ans;
}
