#include <bits/stdc++.h>
#define fr first
#define sc second
#define pii pair<int, int>
#define pb push_back
#define szof(s) (int)s.size()
#define all(s) s.begin(), s.end()
#define fastInp ios_base::sync_with_stdio(0); cin.tie(0);
using namespace std;
const int INF = 1e9;
vector <char> vec = {'J', 'O', 'I', '#'};
int cnt[1000];
int n, k;
string s;
void gg() {
cout << -1 << endl;
exit(0);
}
bool can(int l, int r, int k) {
int cur = 0, count = 0;
for (int i = l; i <= r; i++) {
if (s[i] == vec[cur]) {
count++;
}
if (count == k) {
count = 0;
cur++;
}
}
return (cur == 3);
}
signed main() {
fastInp;
cin >> n >> k;
cin >> s;
int ans = INF;
for (int l = 0; l < n; l++) {
if (can(l, n - 1, k)) {
int bl = l, br = n - 1;
while (br - bl > 1) {
int bmid = (bl + br) >> 1;
if (can(l, bmid, k)) {
br = bmid;
} else {
bl = bmid;
}
}
ans = min(ans, (br - l + 1) - k * 3);
}
}
if (ans == INF) {
gg();
}
cout << ans << endl;
}
/*
10 2
OJIJOIOIIJ
*/
