#include <bits/stdc++.h>
using namespace std;
#define int long long
#define FOR(i,a,b) for (int i = (a); i < (b); ++i)
#define pb push_back
#define all(a) a.begin(), a.end()
#define endl "\n"
void printVector(vector<int> a){
for (auto x: a) cout << x << " ";
cout << endl;
}
void solve(){
int n, k; cin >> n >> k;
string s; cin >> s;
vector<int> j;
vector<int> o;
vector<int> i;
FOR(b,0,n){
if (s[b] == 'J') j.pb(b);
else if (s[b] == 'O') o.pb(b);
else i.pb(b);
}
FOR(b,0,n){
j.pb(n+1);
o.pb(n+1);
i.pb(n+1);
}
int ans = n+1;
bool can = false;
if (j.size() == 0 || o.size() == 0 || i.size() == 0){
cout << -1 << endl;
return;
}
FOR(b,0,1){
auto ind = (lower_bound(all(j), b) - j.begin());
if (j[ind] < b) continue;
int jc = j[ind+k-1];
if (jc > n) break;
// cout << ind << jc << endl;
auto ind1 = (lower_bound(all(o), jc) - o.begin());
if (o[ind1] < jc) continue;
int oc = o[ind1+k-1];
if (oc > n) break;
// cout << ind1 << oc << endl;
auto ind2 = (lower_bound(all(i), oc) - i.begin());
if (i[ind2] < oc) continue;
int ic = i[ind2+k-1];
// cout << ic << endl;
if (ic < n){
can = true;
ans = min(ans, ic-j[ind]+1-3*k);
}
}
if (can) cout << ans << endl;
else cout << -1 << endl;
}
int32_t main(){
ios::sync_with_stdio(false);cin.tie(nullptr);
int t = 1; // cin >> t;
while (t--) solve();
}
