#include <bits/stdc++.h>
using namespace std;
int N, K;
string s;
int main() {
cin >> N >> K >> s;
vector<int> J, O, I;
for (int i = 0; i < (int)s.size(); i++) {
if (s[i] == 'J') J.push_back(i+1);
else if (s[i] == 'O') O.push_back(i+1);
else I.push_back(i+1);
}
O.push_back(INT_MAX);
I.push_back(INT_MAX);
int ans = INT_MAX;
for (int i = 0; i+K-1 < (int)J.size(); i++) {
int cur = 0;
vector<int> arr;
for (int j = i; j < i+K; j++) {
if (!arr.empty()) cur += J[j]-arr.back()-1;
arr.push_back(J[j]);
}
int u = upper_bound(O.begin(), O.end(), arr.back())-O.begin();
if (u == INT_MAX) break;
bool flag = false;
for (int j = u; j < u+K; j++) {
if (O[j] == INT_MAX) {
flag = true;
break;
}
cur += O[j]-arr.back()-1;
arr.push_back(O[j]);
}
if (flag) break;
u = upper_bound(I.begin(), I.end(), arr.back())-I.begin();
if (u == INT_MAX) break;
for (int j = u; j < u+K; j++) {
if (I[j] == INT_MAX) {
flag = true;
break;
}
cur += I[j]-arr.back()-1;
arr.push_back(I[j]);
}
if (flag) break;
ans = min(ans, cur);
}
if (ans == INT_MAX) ans = -1;
cout << ans << "\n";
}
