#include <bits/stdc++.h>
using namespace std;
using i64 = long long;
int main(){
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n, k;
cin >> n >> k;
string s;
cin >> s;
int ans = INT_MAX;
bool ok = false;
vector<int> pj(n + 1, 0);
vector<int> po(n + 1, 0);
vector<int> pi(n + 1, 0);
for (int i = 0; i < n; i++){
pj[i + 1] = pj[i] + (s[i] == 'J');
}
for (int i = 0; i < n; i++){
po[i + 1] = po[i] + (s[i] == 'O');
}
for (int i = 0; i < n; i++){
pi[i + 1] = pi[i] + (s[i] == 'I');
}
auto sum = [&](vector<int> &p, int l, int r) -> int{
return p[r + 1] - p[l];
};
for (int i = 0; i < n; i++){
if (s[i] == 'J'){
// start the joi here
// get the J
int l = i;
int lo = i, hi = n - 1;
// nnnnnnyyy
while (lo < hi){
int mid = (lo + hi) >> 1;
if (sum(pj, l, mid) >= k){
hi = mid;
}
else{
lo = mid + 1;
}
}
if (sum(pj, l, lo) < k){
continue;
}
l = lo + 1;
lo = l;
hi = n - 1;
//get the O
while (lo < hi){
int mid = (lo + hi) >> 1;
if (sum(po, l, mid) >= k){
hi = mid;
}
else{
lo = mid + 1;
}
}
if (sum(po, l, lo) < k){
continue;
}
l = lo + 1;
lo = l;
hi = n - 1;
//get the I
while (lo < hi){
int mid = (lo + hi) >> 1;
if (sum(pi, l, mid) >= k){
hi = mid;
}
else{
lo = mid + 1;
}
}
if (sum(pi, l, lo) < k){
continue;
}
ok = true;
ans = min(ans, lo - i + 1 - (k * 3));
}
}
if (!ok){
cout << "-1" << '\n';
}
else{
cout << ans << '\n';
}
}
