#include <bits/stdc++.h>
using namespace std;
int main()
{
ios::sync_with_stdio(false); cin.tie(0);
int n, k; cin >> n >> k;
string s; cin >> s;
int st = -1, ed = -1;
for (int i = 0; i < s.size(); ++i) {
if (s[i] == 'J' && st == -1) st = i;
if (s[i] == 'I') ed = i;
}
int ops = 0;
int cntJ = 0, cntO = 0, cntI = 0;
for (int i = st; i <= ed; ++i)
{
if (cntJ < k) {
if (s[i] != 'J') ops++;
else cntJ++;
}
else if (cntJ == k && cntO < k) {
if (s[i] != 'O') ops++;
else cntO++;
}
else if (cntJ == k && cntO == k && cntI < k) {
if (s[i] != 'I') ops++;
else cntI++;
}
}
if (cntJ < k || cntO < k || cntI < k) cout << -1;
else cout << ops;
return 0;
}
