#include <bits/stdc++.h>
#define int long long
#define fi first
#define se second
#define mp make_pair
#define Sz(x) (int)x.size()
using namespace std;
const int N = 2e5 + 5;
int n, k, suf[N];
string s;
void solve() {
cin >> n >> k >> s;
vector <int> sj, so, si;
for (int i = 0; i < n; i++) {
if (s[i] == 'J') sj.push_back(i);
else if (s[i] == 'O') so.push_back(i);
else si.push_back(i);
}
reverse(so.begin(), so.end());
reverse(si.begin(), si.end());
int ans = INT_MAX;
for (int i = 0; i + k - 1 < Sz(sj); i++) {
while (Sz(so) > 0 && so.back() <= sj[i + k - 1]) so.pop_back();
if (Sz(so) < k) break;
while (Sz(si) > 0 && si.back() <= so[Sz(so) - k]) si.pop_back();
if (Sz(si) < k) break;
int l = sj[i], r = si[Sz(si) - k];
ans = min(ans, n - (l + (n - r - 1)) - 3 * k);
}
if (ans == INT_MAX) cout << -1;
else cout << ans;
}
signed main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
int tt = 1;
// cin >> tt;
while (tt--) {
solve();
// cout << '\n';
}
}
