#include <bits/stdc++.h>
#define ll long long
using namespace std;
char C[3] = {'J', 'O', 'I'};
int main() {
ios::sync_with_stdio(0), cin.tie(0);
int n, k; cin >> n >> k;
map <char, vector <int>> P;
string s; cin >> s;
s = " " + s;
for (int i = 1; i <= n; i++) P[s[i]].emplace_back(i);
for (auto &e : C) if ((int) P[e].size() < k) { cout << -1 << '\n'; exit(0); }
deque <int> dq;
int mn = INT_MAX;
for (auto &e : P['O']) {
dq.emplace_back(e);
if ((int) dq.size() > k) dq.pop_front();
if ((int) dq.size() < k) continue;
int L = dq.front(), R = dq.back();
int it1 = upper_bound(P['J'].begin(), P['J'].end(), L) - P['J'].begin() - k;
int it2 = upper_bound(P['I'].begin(), P['I'].end(), R) - P['I'].begin() + k - 1;
if (it1 < 0 || it2 >= (int) P['I'].size()) continue;
mn = min(mn, P['I'][it2] - P['J'][it1] + 1 - 3 * k);
}
cout << (mn == INT_MAX ? -1 : mn) << '\n';
}
