#include <bits/stdc++.h>
using namespace std;
using ll = long long;
ll INF = 1e18;
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
ll n, k;
cin >> n >> k;
vector<ll> v1, v2, v3;
string s;
cin >> s;
for (ll i = 0; i < n; i++) {
if (s[i] == 'J') {
v1.push_back(i);
}
if (s[i] == 'O') {
v2.push_back(i);
}
if (s[i] == 'I') {
v3.push_back(i);
}
}
ll ans = INF;
for (ll i = 0; i < (ll) v2.size() - k + 1; i++) {
ll l = v2[i];
ll r = v2[i + k - 1];
ll h1 = upper_bound(v3.begin(), v3.end(), r) - v3.begin() + k - 1;
if (h1 >= v3.size()) {
continue;
}
h1 = v3[h1];
ll h2 = upper_bound(v1.begin(), v1.end(), l) - v1.begin() - k;
if (h2 < 0) {
continue;
}
h2 = v1[h2];
ans = min(ans, (h1 - h2 + 1 - 3 * k));
}
if (ans == INF) {
cout << -1;
return 0;
}
cout << ans;
}
