#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <cmath>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <unordered_map>
#include <unordered_set>
#include <bitset>
#include <numeric>
#include <functional>
#include <iomanip>
#include <sstream>
#include <numeric>
#define endl "\n"
#define int long long
#define pb push_back
#define fi first
#define se second
#pragma GCC optimize("O3")
//#pragma GCC optimize("Ofast")
//pragma GCC optimize("unroll-loops")
using namespace std;
int INF=1e18;
int MOD=1e9+7;
int MOD2=1e9;
int ABCMOD=998244353;
//mac olunca template kullanmak zorundayim :/
inline void solve(){
int n,k; cin>>n>>k;
string s; cin>>s;
vector<int> posj(n,-1),poso(n,-1),posi(n,-1);
int J=0,O=0,I=0;
for(int i=0;i<n;i++){
if(s[i]=='J'){
posj[J]=i;
J++;
}
else if(s[i]=='O'){
poso[O]=i;
O++;
}
else{
posi[I]=i;
I++;
}
}
/*
for(int i=0;i<n;i++) cout<<posj[i]<<" ";
cout<<endl;
for(int i=0;i<n;i++) cout<<poso[i]<<" ";
cout<<endl;
for(int i=0;i<n;i++) cout<<posi[i]<<" ";
cout<<endl; */
if(J<k || O<k || I<k){
cout<<"-1"<<endl; return;
}
vector<int> js(J-k+1), os(O-k+1), is(I-k+1);
vector<int> je(J-k+1), oe(O-k+1), ie(I-k+1);
for(int i=0;i<J-k+1;i++){
js[i]=posj[i];
je[i]=posj[i+k-1];
}
for(int i=0;i<O-k+1;i++){
os[i]=poso[i];
oe[i]=poso[i+k-1];
//cout<<os[i]<<" "<<oe[i]<<endl;
}
for(int i=0;i<I-k+1;i++){
is[i]=posi[i];
ie[i]=posi[i+k-1];
//cout<<is[i]<<" "<<ie[i]<<endl;
}
int mn=INF;
for(int i=0;i<J-k+1;i++){
auto oloc=upper_bound(os.begin(),os.end(),je[i])-os.begin();
if(oloc>=O-k+1) break;
auto iloc=upper_bound(is.begin(),is.end(),oe[oloc])-is.begin();
if(iloc>=I-k+1) break;
int len=ie[iloc]-js[i]+1;
mn=min(mn,len);
}
int ans=mn-(3*k);
if(ans>=INF/2) cout<<"-1"<<endl;
else cout<<ans<<endl;
}
signed main(){
ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);
//freopen("hps.in", "r", stdin); freopen("hps.out", "w", stdout);
int T=1; //cin>>T;
while (T--){
solve();
}
}
