#include <bits/stdc++.h>
using namespace std;
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n, k;
string s;
cin >> n >> k >> s;
vector<vector<int>> p(3, vector<int>(n + 1));
string t = "JOI";
for (int i = 0; i < 3; i++) {
for (int j = 0; j < n; j++) {
p[i][j + 1] = p[i][j] + (s[j] == t[i]);
}
}
vector<int> pos, ls, rs;
for (int i = 0; i < n; i++) {
if (s[i] == 'O') {
pos.push_back(i);
} else if (s[i] == 'J') {
ls.push_back(i);
} else {
rs.push_back(i);
}
}
int ans = n;
for (int i = 0; i + k <= pos.size(); i++) {
int l = ranges::upper_bound(ls, pos[i]) - ls.begin() - k;
if (l < 0) continue;
int r = ranges::upper_bound(rs, pos[i + k - 1]) - rs.begin() + k - 1;
if (r >= rs.size()) continue;
ans = min(ans, rs[r] - ls[l] + 1 - 3 * k);
}
cout << (ans == n ? -1 : ans) << '\n';
return 0;
}
