#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define F first
#define S second
#define pb push_back
#define pii pair<int, int>
const int N = 2e5 + 5;
int n, k, lef[N][3];
string s;
deque<int> q[3];
string t;
signed main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cin >> n >> k;
cin >> s;
s = " " + s;
t = "JOI";
int ans = n + 1;
for (int i = 1; i <= n; i++) {
for (int j = 0; j < 3; j++) {
lef[i][j] = lef[i - 1][j];
if (s[i] != t[j]) continue;
q[j].push_front(i);
while(q[j].size() > k) q[j].pop_back();
if (q[j].size() == k) lef[i][j] = q[j].back();
else {
lef[i][j] = 0;
}
// for (auto it : q) cout << it << ' ';
// cout << s[i] << ' ' << t[j] << ' ' << lef[i][j] << '\n';
}
int l = i;
bool ok = 1;
for (int j = 2; j >= 0; j--) {
if (lef[l][j] > 0) {
l = lef[l][j];
}
else {
ok = 0;
break;
}
}
if (ok) {
// cout << l << ' ' << i << endl;
ans = min(ans, i - l + 1);
}
}
if (ans == n + 1) {
cout << -1;
}
else cout << ans - 3 * k;
}
