#include <bits/stdc++.h>
using namespace std;
int find_pos(const vector<int>& v, int pos, int k) {
auto it = lower_bound(v.begin(), v.end(), pos); // Binary search for lower bound
int cnt = 0;
if (it != v.end()) cnt++; // if there is at least one element greater than or equal to pos
while (it != v.end() && cnt != k) {
cnt++;
it++;
}
if (cnt == k && it != v.end()) return *it;
else return -1;
}
int main() {
int n, k;
cin >> n >> k;
string a;
cin >> a;
vector<int> b[3]; // Using vectors for 'J', 'O', and 'B' positions
// Store positions of J, O, and B
for (int i = 0; i < n; i++) {
if (a[i] == 'J') b[0].push_back(i);
else if (a[i] == 'O') b[1].push_back(i);
else b[2].push_back(i);
}
// Sort the vectors for binary search
for (int i = 0; i < 3; i++) {
sort(b[i].begin(), b[i].end());
}
int mnm = 1e9;
// Try to find the smallest sequence of 'J', 'O', 'B'
for (int i = 0; i < n; i++) {
if (a[i] == 'J') {
int pos1 = find_pos(b[0], i, k);
if (pos1 != -1) {
int pos2 = find_pos(b[1], pos1, k);
if (pos2 != -1) {
int pos3 = find_pos(b[2], pos2, k);
if (pos3 != -1) {
int loc = pos3 - i + 1;
int cr = 3 * k;
if (loc > 0 && cr > 0 && loc - cr >= 0) {
mnm = min(mnm, loc - cr);
}
}
}
}
}
}
// Output result
if (mnm != 1e9) {
cout << mnm;
} else {
cout << -1;
}
return 0;
}
