#include <bits/stdc++.h>
#define ll long long
using namespace std;
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
ll n,k;
cin >> n >> k;
string s;
cin >> s;
vector<ll> idxj, idxo, idxi;
for (int i=0; i<n; i++) {
if (s[i]=='J') idxj.push_back(i);
if (s[i]=='O') idxo.push_back(i);
if (s[i]=='I') idxi.push_back(i);
}
if (idxj.size() < k || idxo.size() < k || idxi.size() < k) {
cout << -1;
return 0;
}
ll j=0, o=0;
ll res = INT_MAX;
for (int i=0; i<=idxj.size()-k; i++) {
while(j<idxo.size() && idxo[j]<idxj[i+k-1]) j++;
while(o<idxi.size() && idxi[o]<idxo[j+k-1]) o++;
if (i+k>idxj.size() || o+k>idxi.size() || j+k>idxo.size()) break;
if (idxj.size()-i>=k && idxo.size()-j>=k && idxi.size()-o>=k) {
ll o1 = idxi[o+k-1];
ll val = (o1-idxj[i]+1)-(3*k);
res = min(res, val);
} else {
break;
}
}
cout << (res==INT_MAX ? -1 : res);
}