#include <iostream>
#include <string>
#include <queue>
using namespace std;
int n,k;
void balansuj(vector<int> &zrodlo, vector<int> &ujscie, int &zp, int &up){
if(zp+k-1>=zrodlo.size()){
up=ujscie.size();
return;
}
while(up<ujscie.size()&&ujscie[up]<zrodlo[zp+k-1]) up++;
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
string s;
cin>>n>>k>>s;
int b=0,e=0,res=n+1;
vector<int> jq,oq,iq;
int jp=0,op=0,ip=0;
while(b<=e){
if(jq.size()-jp>=k&&oq.size()-op>=k&&iq.size()-ip>=k){
res=min(res,e-b-3*k);
if(s[b]=='J') jp++;
else if(s[b]=='O'&&op<oq.size()&&oq[op]==b) op++;
else if(ip<iq.size()&&iq[ip]==b) ip++;
balansuj(jq,oq,jp,op);
balansuj(oq,iq,op,ip);
b++;
}
else{
if(e==n) break;
if(s[e]=='J') jq.push_back(e);
else if(s[e]=='O') oq.push_back(e);
else iq.push_back(e);
balansuj(jq,oq,jp,op);
balansuj(oq,iq,op,ip);
e++;
}
}
if(res==n+1) printf("-1");
else printf("%d",res);
return 0;
}
