#include "bits/stdc++.h"
// @JASPER'S BOILERPLATE
using namespace std;
using ll = long long;
#define fi first
#define se second
#ifdef JASPER
#include "debug.h"
#else
#define debug(...) 166
#endif
using pii = pair < int, int >;
const int INF = 1e9;
const int MOD = 1e9 + 7;
const int N = 2e5 + 5;
int n, k;
string s;
deque <int> q[3];
void run_case() {
cin >> n >> k >> s;
for (int i = 0; i < n; ++i) {
if (s[i] == 'J') q[0].push_back(i);
if (s[i] == 'O') q[1].push_back(i);
if (s[i] == 'I') q[2].push_back(i);
}
int ans = INF;
while ((int) q[0].size() >= k) {
// delete all Os sitting front of k Js
while ((int) q[1].size() > k && q[1].front() < q[0][k - 1])
q[1].pop_front();
// delete all Is front of k Os
while ((int) q[2].size() > k && (int) q[1].size() >= k && q[2].front() < q[1][k - 1])
q[2].pop_front();
if (q[0][k - 1] < q[1][0] && q[1][k - 1] < q[2][0]) {
int l = q[0].front();
int r = q[2][k - 1];
// debug(l, r);
// minimum operation 3
ans = min(ans, r - l + 1 - 3 * k);
}
q[0].pop_front();
}
if (ans == INF) ans = -1;
cout << ans << "\n";
}
signed main() {
cin.tie(0) -> sync_with_stdio(0);
int Test = 1;
//cin >> Test;
for (int test = 1; test <= Test; test++){
run_case();
}
}
