#include<bits/stdc++.h>
#define int long long
#define nl endl
using namespace std;
signed main(){
ios_base::sync_with_stdio(0);
cin.tie(NULL);cout.tie(NULL);
int n, k;
cin>>n>>k;
string s; cin>>s;
vector<int> Jcnt(n), Ocnt(n), Icnt(n);
Jcnt[0] = (s[0] == 'J'), Ocnt[0] = (s[0] == 'O'), Icnt[0] = (s[0] == 'I');
for(int i=1; i<n; i++){
Jcnt[i] = Jcnt[i-1] + (s[i] == 'J');
Ocnt[i] = Ocnt[i-1] + (s[i] == 'O');
Icnt[i] = Icnt[i-1] + (s[i] == 'I');
}
int ans = 1e18;
for(int i=0; i<n; i++){
int Jstart = lower_bound(Jcnt.begin(), Jcnt.end(), i+1) - Jcnt.begin();
int Jend = lower_bound(Jcnt.begin(), Jcnt.end(), i + k) - Jcnt.begin();
if(Jend >= n) continue;
int Oend = lower_bound(Ocnt.begin(), Ocnt.end(), Ocnt[Jend] + k) - Ocnt.begin();
if(Oend >= n) continue;
int Iend = lower_bound(Icnt.begin(), Icnt.end(), Icnt[Oend] + k) - Icnt.begin();
if(Iend >= n) continue;
ans = min(ans, (Iend - Jstart + 1) - (3*k));
}
if(ans == 1e18) cout<<-1;
else cout<<ans;
return 0;
}
