#include <bits/stdc++.h>
using namespace std;
#define ll long long
struct Node {
ll j, o, i, idx;
Node() {}
Node(ll j, ll o, ll i, ll idx) : j(j), o(o), i(i), idx(idx) {}
};
const ll N = 2e5;
Node x[N + 1];
ll find_where_appears(char c, ll k, ll st, ll en) {
ll ost = st;
ll ans = en + 1;
while (st <= en) {
ll m = (st + en) / 2;
ll val;
if (c == 'J') {
val = x[m].j - x[ost - 1].j;
} else if (c == 'O') {
val = x[m].o - x[ost - 1].o;
} else {
val = x[m].i - x[ost - 1].i;
}
if (val >= k) {
ans = m;
en = m - 1;
} else {
st = m + 1;
}
}
return ans;
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
ll n, k;
string s;
cin >> n >> k >> s;
s = " " + s;
x[0].j = x[0].o = x[0].i = x[0].idx = 0;
for (ll i = 1; i <= n; ++i) {
x[i] = x[i - 1];
x[i].j += s[i] == 'J';
x[i].o += s[i] == 'O';
x[i].i += s[i] == 'I';
x[i].idx = i;
}
ll ans = LLONG_MAX;
for (ll j = 1; j <= n; ++j) {
if (s[j] != 'J') {
continue;
}
ll idx = find_where_appears('J', k, j, n);
idx = find_where_appears('O', k, idx, n);
idx = find_where_appears('I', k, idx, n);
if (idx == n + 1) {
continue;
}
ans = min(ans, idx - j + 1 - 3 * k);
}
if (ans == LLONG_MAX) {
cout << "-1\n";
} else {
cout << ans << "\n";
}
}
