#include <bits/stdc++.h>
using namespace std;
map <char, vector <int>> pos;
int n, k;
bool check(int st, int ed) {
auto it = lower_bound(pos['J'].begin(), pos['J'].end(), st) - pos['J'].begin();
it += k - 1;
if (it >= (int)pos['J'].size())
return false;
int p = pos['J'][it];
it = lower_bound(pos['O'].begin(), pos['O'].end(), p) - pos['O'].begin();
it += k - 1;
if (it >= (int)pos['O'].size())
return false;
p = pos['O'][it];
it = lower_bound(pos['I'].begin(), pos['I'].end(), p) - pos['I'].begin();
it += k - 1;
if (it >= (int)pos['I'].size())
return false;
return pos['I'][it] <= ed;
}
int main() {
cin >> n >> k;
string s;
cin >> s;
s = "0" + s;
for (int i = 1; i <= n; i++) {
pos[s[i]].push_back(i);
}
int ans = -1;
for (int i = 1; i <= n; i++) {
if (s[i] != 'J')
continue;
int lo = i, hi = n+1;
int j, o, I;
while (lo < hi) {
int mid = lo + (hi - lo) / 2;
if (check(i, mid))
hi = mid;
else
lo = mid + 1;
}
if (check(i, hi)) {
if (ans == -1)
ans = n - k;
ans = min(ans, (hi - i + 1) - (3 * k));
}
}
cout << ans << '\n';
}
