#include <bits/stdc++.h>
#define int long long
using namespace std;
signed main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
int n, k;
cin >> n >> k;
vector<int> a(n + 1);
vector<int> idx[4];
for(int i = 1; i <= n; i++) {
char x;
cin >> x;
if(x == 'J') {
a[i] = 1;
}
else if(x == 'O') {
a[i] = 2;
}
else {
a[i] = 3;
}
idx[a[i]].push_back(i);
}
vector<int> cnt1(n + 1, 0);
vector<int> cnt2(n + 1, 0);
vector<int> cnt3(n + 1, 0);
for(int i = 1; i <= n; i++) {
cnt1[i] = cnt1[i - 1];
cnt2[i] = cnt2[i - 1];
cnt3[i] = cnt3[i - 1];
if(a[i] == 1) {
cnt1[i] += 1;
}
else if(a[i] == 2) {
cnt2[i] += 1;
}
else {
cnt3[i] += 1;
}
}
int ans = n + 1;
for(int i = 0; i + k - 1 < idx[1].size(); i++) {
int l1 = idx[1][i];
int r1 = idx[1][i + k - 1];
int l2 = - 1;
int r2 = - 1;
for(int j = 0; j + k - 1 < idx[2].size(); j++) {
if(idx[2][j] >= r1) {
l2 = idx[2][j];
r2 = idx[2][j + k - 1];
}
}
int l3 = - 1;
int r3 = - 1;
for(int j = 0; j + k - 1 < idx[3].size(); j++) {
if(idx[3][j] >= r2) {
l3 = idx[3][j];
r3 = idx[3][j + k - 1];
}
}
if(l2 == - 1 || l3 == - 1) {
continue;
}
ans = min(ans, r3 - l1 + 1 - 3 * k);
}
if(ans == n + 1) {
cout << - 1;
}
else {
cout << ans;
}
}
