#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define endl "\n"
#define inf ((int)2e9)
int main() {
ios::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
cout << fixed << setprecision(0);
int n,k;
cin >> n >> k;
string s;
cin >> s;
vector<int> J(n+1),O(n+1),I(n+1);
for (int i = 1; i <= n; i++) {
J[i] = J[i-1] + (s[i-1] == 'J');
O[i] = O[i-1] + (s[i-1] == 'O');
I[i] = I[i-1] + (s[i-1] == 'I');
}
// min cost to place K js, if first O is at I
// vector<int> jc(n+1, inf);
int ans = inf;
for (int i = k+1; i <= n; i++) {
if (J[i-1] < k || s[i-1] != 'O') {
continue;
}
int ind = upper_bound(J.begin(), J.end(), J[i-1]-k) - J.begin();
// cout << i << " " << ind << endl;
int jc = (i - ind) - k;
// cout << i << " " << jc[i] << endl;
int indo = lower_bound(O.begin(), O.end(), O[i]+k-1) - O.begin();
if (indo == n+1) {
continue;
}
int oc = (indo - i + 1) - k;
// cout << i << " " << indo << endl;
if (I[n] - I[indo] < k) {
continue;
}
int indi = lower_bound(I.begin(), I.end(), I[indo]+k) - I.begin();
// cout << indo << " " << indi << endl;
int ic = (indi-indo) - k;
ans = min(ans, jc + oc + ic);
}
cout << (ans == inf ? -1 : ans);
}
