#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
using namespace std;
/**
* JOI 2020 Final - JOI-string
* Vaqt murakkabligi: O(N)
*/
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
int N, K;
string S;
cin >> N >> K >> S;
vector<int> posJ, posO, posI;
for (int i = 0; i < N; i++) {
if (S[i] == 'J') posJ.push_back(i);
else if (S[i] == 'O') posO.push_back(i);
else if (S[i] == 'I') posI.push_back(i);
}
// Agar harflar yetarli bo'lmasa
if (posJ.size() < K || posO.size() < K || posI.size() < K) {
cout << -1 << endl;
return 0;
}
int min_op3 = 2e9;
// Har bir mumkin bo'lgan K ta 'J' blokidan boshlaymiz
for (int i = 0; i + K <= posJ.size(); i++) {
int endJ = posJ[i + K - 1];
// 'J' lardan keyin keladigan birinchi 'O' indeksini topamiz
auto itO = lower_bound(posO.begin(), posO.end(), endJ);
int idxO = distance(posO.begin(), itO);
if (idxO + K > posO.size()) continue;
int endO = posO[idxO + K - 1];
// 'O' lardan keyin keladigan birinchi 'I' indeksini topamiz
auto itI = lower_bound(posI.begin(), posI.end(), endO);
int idxI = distance(posI.begin(), itI);
if (idxI + K > posI.size()) continue;
int endI = posI[idxI + K - 1];
// Umumiy oraliq: [posJ[i], endI]
// Bu oraliqdagi barcha keraksiz belgilar Op3 orqali o'chiriladi
int current_total_len = endI - posJ[i] + 1;
min_op3 = min(min_op3, current_total_len - 3 * K);
}
if (min_op3 > N) cout << -1 << endl;
else cout << min_op3 << endl;
return 0;
}
