#include <bits/stdc++.h>
#define int long long
#define pb push_back
#define fs first
#define sc second
using namespace std;
signed main() {
int n, k;
string s;
cin >> n >> k >> s;
vector<int> posj, poso, posi;
for (int i = 0; i < n; ++i) {
if (s[i] == 'J') posj.pb(i);
else if (s[i] == 'O') poso.pb(i);
else if (s[i] == 'I') posi.pb(i);
}
int ans = 1e9;
for (int j = 0; j + k - 1 < posj.size(); ++j) {
int jend = posj[j + k - 1];
auto o = lower_bound(poso.begin(), poso.end(), jend + 1);
if (o + k - 1 >= poso.end()){
continue;
}
int oend = *(o + k - 1);
auto i = lower_bound(posi.begin(), posi.end(), oend + 1);
if (i + k - 1 >= posi.end()){
continue;
}
int iend = *(i + k - 1);
int len = iend - posj[j] + 1;
int cost = len - 3 * k;
ans = min(ans, cost);
}
if(ans == 1e9){
cout << -1;
return 0;
}
cout << ans;
}
