#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define vi vector<long long>
#define all(x) x.begin(), x.end()
#define endl "\n"
void solution();
int main(){
ios::sync_with_stdio(false);
cin.tie(nullptr);
solution();
return 0;
}
void solution() {
ll n, k; cin >> n >> k;
string s; cin >> s;
while (s.size() > 0 && s.back() != 'I') s.pop_back();
reverse(all(s));
while (s.size() > 0 && s.back() != 'J') s.pop_back();
reverse(all(s));
n = s.size();
if (!n) {
cout << -1 << endl;
return;
}
s = '0'+s;
vi ps(n+1), suf(n+2);
for (int i = 1; i <= n; i++) ps[i] = ps[i-1] + (s[i] == 'J');
for (int i = n; i >= 1; i--) suf[i] = suf[i+1] + (s[i] == 'I');
// cout << s << endl;
ll ans = 1e18;
int j = 1;
int cnt = 0;
for (int i = 1; i <= n; i++) {
j = max(j, i);
if (s[i] != 'O') continue;
while (j <= n && cnt < k) {
cnt += (s[j] == 'O');
j++;
}
if (cnt != k) continue;
cnt--;
ll temp = j-i-k;
int p = 1, q = i-1;
int d = -1;
while (p <= q) {
int mid = (p + q)/2;
if (ps[i]-ps[mid-1] >= k) {
d = mid;
p = mid+1;
} else q = mid-1;
}
if (d == -1) continue;
temp += i-d-k;
d = -1;
p = j, q = n;
while (p <= q) {
int mid = (p + q)/2;
if (suf[j]-suf[mid+1] >= k) {
d = mid;
q = mid-1;
} else p = mid+1;
}
if (d == -1) continue;
temp += d - (j-1) - k;
ans = min(ans, temp);
}
cout << (ans == 1e18? -1 : ans) << endl;
}
