#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
#include "debug.h"
#else
#define dbg(...) 42
#endif
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n, k;
cin >> n >> k;
string a;
cin >> a;
int l = 0, r = n - 1;
while (a[l] != 'J') l++;
while (a[r] != 'I') r--;
string s;
for (int i = 0; i < 3; i++) {
for (int j = 0; j < k; j++) {
if (i == 0) s += 'J';
if (i == 1) s += 'O';
if (i == 2) s += 'I';
}
}
int id = 0;
int res = 0;
for (int i = l; i <= r; i++) {
if (a[i] == s[id]) {
id++;
}
else {
res++;
}
}
if (id != 3 * k) {
cout << -1 << '\n';
}
else {
cout << res << '\n';
}
return 0;
}
