#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define int ll
const ll N = 2e5 + 5, inf = 1e18, mod = 1e9 + 7, block = 320, lim = 19;
int n, k;
string s;
struct item {
int j, o, i;
} nxt[N + 2];
int up[N + 2][lim + 1];
int cnt[N + 2][lim + 1][5];
char whatc[4] = {'?', 'J', 'O', 'I'};
int find_next(int u, int type) {
int have = k;
if (u <= n && s[u] == whatc[type]) {
have--;
if (have == 0) return u;
}
for (int j = lim; j >= 0; j--) {
int v = up[u][j];
if (v <= n && cnt[u][j][type] < have) {
have -= cnt[u][j][type];
u = v;
}
}
u = up[u][0];
if (u <= n && s[u] == whatc[type] && have == 1) return u;
return -1;
}
bool check(int mid) {
for (int j = nxt[0].j; j <= n; j = nxt[j].j) {
int First = find_next(j, 1);
if (First == -1) return false;
int Second = find_next(nxt[First].o, 2);
if (Second == -1) return false;
int Third = find_next(nxt[Second].i, 3);
if (Third == -1) return false;
if ((Third - j + 1) - 3 * k <= mid) return true;
}
return false;
}
signed main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
cin >> n >> k >> s;
s = "#" + s;
int nj = n + 1, no = n + 1, ni = n + 1;
for (int i = n; i >= 1; i--) {
nxt[i] = {nj, no, ni};
if (s[i] == 'J') nj = i;
else if (s[i] == 'O') no = i;
else ni = i;
}
nxt[0] = {nj, no, ni};
nxt[n + 1] = {n + 1, n + 1, n + 1};
for (int i = 0; i <= n + 1; i++) {
if (i >= 1 && i <= n) {
if (s[i] == 'J') up[i][0] = nxt[i].j;
else if (s[i] == 'O') up[i][0] = nxt[i].o;
else up[i][0] = nxt[i].i;
} else up[i][0] = i;
for (int t = 1; t <= 3; t++) cnt[i][0][t] = 0;
if (i >= 1 && i <= n) {
int x = up[i][0];
if (x <= n) {
cnt[i][0][1] = (s[x] == 'J');
cnt[i][0][2] = (s[x] == 'O');
cnt[i][0][3] = (s[x] == 'I');
}
}
}
for (int j = 1; j <= lim; j++) {
for (int i = 0; i <= n + 1; i++) {
int x = up[i][j - 1];
up[i][j] = up[x][j - 1];
for (int t = 1; t <= 3; t++) {
cnt[i][j][t] = cnt[i][j - 1][t] + cnt[x][j - 1][t];
}
}
}
int l = 0, r = n, ans = -1;
while (l <= r) {
int mid = (l + r) / 2;
if (check(mid)) {
ans = mid;
r = mid - 1;
} else l = mid + 1;
}
cout << ans << '\n';
return 0;
}
