#include<bits/stdc++.h>
using namespace std;
int n,k;
signed main(){
cin>>n>>k;
string s;
cin>>s;
vector<int> j1;
vector<int> o1;
vector<int> i1;
for (int i = 0;i<n;i++){
if (s[i]=='J') j1.push_back(i);
else if (s[i]=='O') o1.push_back(i);
else i1.push_back(i);
}
int ans = 1e9;
for (int i = 0;i<j1.size();i++){
if (j1.size()<k+i) {break;}
int ans1 = 0;
ans1=ans1+j1[i+k-1]-j1[i]-k+1;
int c = lower_bound(o1.begin(),o1.end(),j1[i+k-1])-o1.begin();
if (o1.size()<c+k){break;}
ans1 = ans1+o1[c+k-1]-o1[c]-k+o1[c]-j1[i+k-1];
int c1 = lower_bound(i1.begin(),i1.end(),o1[c+k-1])-i1.begin();
if (i1.size()<c1+k){break;}
ans1 = ans1+i1[c1+k-1]-i1[c1]-k+i1[c1]-o1[c+k-1];
ans=min(ans,ans1);
}
if (ans!=1e9) cout<<ans;
else cout<<-1;
return 0;
}
