#include<bits/stdc++.h>
using namespace std;
vector<int> place[3];
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(0);
int n, k; string s; cin>>n>>k>>s;
for(int i = 0; i < n; i++){
if(s[i] == 'J') place[0].push_back(i);
if(s[i] == 'O') place[1].push_back(i);
if(s[i] == 'I') place[2].push_back(i);
}
int ans = 1e9;
for(int i = 0; i < n; i++){
int p = i;
bool ok = 1;
for(int j = 0; j <= 2; j++){
p = lower_bound(place[j].begin(), place[j].end(), p) - place[j].begin() + k - 1;
if(p >= place[j].size()){ok = 0; break;}
p = place[j][p] + 1;
}
if(ok == 1) ans = min(ans, p-i-3*k);
}
if(ans <= n) cout<<ans;
else cout<<-1;
}
