#include "bits/stdc++.h"
using namespace std;
using i64 = long long;
using u64 = unsigned long long;
using i32 = int;
using u32 = unsigned;
using i16 = short;
using u16 = unsigned short;
using ld = long double;
using ii = pair<int, int>;
const int inf = 1e9;
void solve() {
int n, k, ans = inf;
string s;
cin >> n >> k >> s;
vector<int> p[3];
for(int i = 0; i < n; ++i) {
if(s[i] == 'J') p[0].push_back(i);
else if(s[i] == 'O') p[1].push_back(i);
else p[2].push_back(i);
}
for(int i = 0; i < n; ++i) {
if(s[i] != 'J') continue;
int last = i;
bool ok = true;
for(int j = 0; j < 3; ++j) {
int l = lower_bound(p[j].begin(), p[j].end(), last) - p[j].begin();
if(l + k > (int)p[j].size()) {
ok = false;
break;
}
last = p[j][l + k - 1];
}
if(!ok) continue;
ans = min(ans, last - i + 1 - 3 * k);
}
if(ans == inf) cout << "-1\n";
else cout << ans << '\n';
}
int main() {
ios_base :: sync_with_stdio(false);
cin.tie(0);
int t = 1;
//cin >> t;
while(t--) solve();
return 0;
}
