#include "bits/stdc++.h"
using namespace std;
#define for_(i, s, e) for (int i = s; i < (int) e; i++)
#define for__(i, s, e) for (ll i = s; i < e; i++)
typedef long long ll;
typedef vector<int> vi;
typedef pair<int, int> ii;
#define endl '\n'
int main() {
#ifdef mlocal
freopen("test.in", "r", stdin);
#endif
ios_base::sync_with_stdio(false);
cin.tie(0);
int n, k; cin >> n >> k;
string s; cin >> s;
vector<vi> pref(n, vi(3));
for_(i, 0, n) {
if (s[i] == 'J') pref[i][0]++;
else if (s[i] == 'O') pref[i][1]++;
else pref[i][2]++;
}
for_(i, 1, n) for_(j, 0, 3) pref[i][j] += pref[i-1][j];
vi pre(n), curr(n);
for_(i, 0, n) pre[i] = i;
for (int j = 2; j >= 0; j--) {
curr.assign(n, n);
int pt = n;
for (int i = n-1; i >= 0; i--) {
while (pt > i and pref[pt-1][j] - (i > 0 ? pref[i-1][j] : 0) >= k) pt--;
curr[i] = pt;
if (pt != n) curr[i] = pre[pt];
}
swap(curr, pre);
}
int ans = n;
for_(i, 0, n) if (pre[i] != n) ans = min(ans, pre[i]-i+1-3*k);
cout << (ans == n ? -1 : ans) << endl;
return 0;
}
