#pragma GCC optimize("O3,inline")
#include <bits/stdc++.h>
using namespace std;
#define int long long
signed main() {
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
int N, K, ans = -1; cin >> N >> K;
string s; cin >> s;
vector<vector<int>> idxs(3);
for (int i = 0; i < N; i++) {
if (s[i] == 'J') idxs[0].push_back(i);
else if (s[i] == 'O') idxs[1].push_back(i);
else idxs[2].push_back(i);
}
if (min(idxs[0].size(), min(idxs[1].size(), idxs[2].size())) < K) {
cout << "-1\n";
return 0;
}
for (int i = 0; i <= N; i++) {
auto iter0 = lower_bound(idxs[0].begin(), idxs[0].end(), i);
if (idxs[0].end() - iter0 < K) break;
int lb = *iter0;
iter0 += K - 1;
auto iter1 = lower_bound(idxs[1].begin(), idxs[1].end(), *iter0);
if (idxs[1].end() - iter1 < K) break;
iter1 += K - 1;
auto iter2 = lower_bound(idxs[2].begin(), idxs[2].end(), *iter1);
if (idxs[2].end() - iter2 < K) break;
iter2 += K - 1;
ans = max(ans, lb + N - 1 - *iter2);
}
if (ans == -1) cout << ans << '\n';
else cout << N - 3 * K - ans << '\n';
}
