//supernova - yunosuke & haruno
#include <bits/stdc++.h>
#define pb push_back
#define fir first
#define sec second
#define int long long
using namespace std;
signed main(){
int n, k; cin>>n>>k;
string s; cin>>s;
vector<int> vj, vo, vi;
vector<int> dj(1, 0), di(1, 0);
for(int i=0; i<n; i++){
if(s[i]=='J') vj.pb(i);
else if(s[i]=='O') vo.pb(i);
else vi.pb(i);
}
if(vo.size()<k || vj.size()<k || vi.size()<k){
cout<<-1;
return 0;
}
for(int i=0; i<vj.size()-1; i++){
dj.pb(vj[i+1]-vj[i]-1);
}
for(int i=0; i<vi.size()-1; i++){
di.pb(vi[i+1]-vi[i]-1);
}
vector<int> pj(dj.size(), 0);
vector<int> pi(di.size(), 0);
for(int i=1; i<dj.size(); i++){
pj[i]=pj[i-1]+dj[i];
}
for(int i=1; i<di.size(); i++){
pi[i]=pi[i-1]+di[i];
}
int ort=0;
for(int i=0; i<k-1; i++){
ort+=vo[i+1]-vo[i]-1;
}
int ans=LLONG_MAX;
for(int i=0; i<=vo.size()-k; i++){
int tmp=ort;
int son=i+k-1;
//sonda o'lar 1 kayıyor
if(i+1<vo.size()){
ort-=vo[i+1]-vo[i]-1;
}
if(son+1<vo.size()){
ort+=vo[son+1]-vo[son]-1;
}
auto sol=lower_bound(vj.begin(), vj.end(), vo[i]);
int r1=sol-vj.begin()-1;
int l1=r1-k+1;
if(l1<0){
continue;
}
tmp+=vo[i]-vj[r1]-1; //sağdaki j ve soldaki o arası
tmp+=pj[r1]-pj[l1];
auto sag=lower_bound(vi.begin(), vi.end(), vo[son]);
int l2=sag-vi.begin();
int r2=l2+k-1;
if(r2>=vi.size()) continue;
tmp+=vi[l2]-vo[son]-1;
tmp+=pi[r2]-pi[l2];
ans=min(ans, tmp);
}
if(ans==LLONG_MAX) cout<<-1;
else cout<<ans;
return 0;
}
