#include <bits/stdc++.h>
using namespace std;
int N, K, A = 1e9, C[5][200005];
char S[200005];
int main() {
ios::sync_with_stdio(0);
cin.tie(0);
cin >> N >> K;
for (int i = 1; i <= N; i++) cin >> S[i];
for (int i = N; i >= 1; i--) {
C[1][i] = C[1][i + 1];
C[2][i] = C[2][i + 1];
C[3][i] = C[3][i + 1];
if (S[i] == 'J') C[1][i]++;
else if (S[i] == 'O') C[2][i]++;
else C[3][i]++;
}
for (int i = 1; i <= N; i++) {
int lo = 1, hi = i - K + 1, ans = -1, cans;
while (lo <= hi) {
int mid = (lo + hi) >> 1;
if (C[3][mid] - C[3][i + 1] >= K) ans = mid, lo = mid + 1;
else hi = mid - 1;
}
if (ans == -1) continue;
lo = 1, hi = ans - K + 1, cans = ans, ans = -1;
while (lo <= hi) {
int mid = (lo + hi) >> 1;
if (C[2][mid] - C[2][cans + 1] >= K) ans = mid, lo = mid + 1;
else hi = mid - 1;
}
if (ans == -1) continue;
lo = 1, hi = ans - K + 1, cans = ans, ans = -1;
while (lo <= hi) {
int mid = (lo + hi) >> 1;
if (C[1][mid] - C[1][cans + 1] >= K) ans = mid, lo = mid + 1;
else hi = mid - 1;
}
if (ans == -1) continue;
A = min(A, i - ans + 1);
}
cout << (A == 1e9 ? -1 : A - 3 * K) << '\n';
}
