#include <bits/stdc++.h>
using namespace std;
#define f first
#define s second
int N, K;
string str;
int main() {
cin >> N >> K >> str;
vector<int> J, O, I;
for (int i = 0; i < (int)str.size(); i++) {
if (str[i] == 'J') {
J.push_back(i+1);
}
else if (str[i] == 'O') {
O.push_back(i+1);
}
else {
I.push_back(i+1);
}
}
vector<int> A[2], B[2], C[2];
for (int i = 0; i+K-1 < (int)J.size(); i++) {
A[0].push_back(J[i]);
A[1].push_back(J[K+i-1]);
}
for (int i = 0; i+K-1 < (int)O.size(); i++) {
B[0].push_back(O[i]);
B[1].push_back(O[K+i-1]);
}
for (int i = 0; i+K-1 < (int)I.size(); i++) {
C[0].push_back(I[i]);
C[1].push_back(I[K+i-1]);
}
int ans = INT_MAX;
for (int a = 0; a < (int)A[0].size(); a++) {
if (upper_bound(B[0].begin(), B[0].end(), A[1][a]) == B[0].end()) break;
int b = upper_bound(B[0].begin(), B[0].end(), A[1][a])-B[0].begin();
if (upper_bound(C[0].begin(), C[0].end(), B[1][b]) == C[0].end()) break;
int c = upper_bound(C[0].begin(), C[0].end(), B[1][b])-C[0].begin();
ans = min(ans, C[1][c]-A[0][a]-3*K+1);
}
if (ans == INT_MAX) ans = -1;
cout << ans << "\n";
}
