#include<bits/stdc++.h>
#define int long long
#define all(v) v.begin(), v.end()
#define SZ(x) (int)x.size()
#define pii pair<int, int>
#define X first
#define Y second
using namespace std;
const int maxn = 2e5 + 10;
const int mod = 1e9 + 7;// 998244353;
const int llmx = 1e18;
void sol(){
int n, k;
cin >> n >> k;
string s; cin >> s;
s = " " + s;
vector< vector< int > > cnt(3);
for(int i = 1; i <= n; ++i){
cnt[s[i] == 'J' ? 0 : s[i] == 'O' ? 1 : 2].push_back(i);
}
int ans = llmx;
auto chk = [&](int l, int r) -> void {
int L = upper_bound(all(cnt[0]), l) - cnt[0].begin() - k;
int R = lower_bound(all(cnt[2]), r) - cnt[2].begin() + k - 1;
if(L < 0 || R >= SZ(cnt[2])) return;
ans = min(ans, n - (cnt[0][L] - 1 + (n - cnt[2][R])) - 3 * k);
};
for(int i = 0; i + k - 1 < SZ(cnt[1]); ++i){
chk(cnt[1][i], cnt[1][i + k - 1]);
}
if(ans == llmx) cout << "-1\n";
else cout << ans << "\n";
}
/*
10 2
OJIJOIOIIJ
2
9 3
JJJOOOIII
0
9 1
IIIOOOJJJ
-1
*/
signed main(){
ios::sync_with_stdio(0), cin.tie(0), cerr.tie(0);
int t = 1; //cin >> t;
while(t--) sol();
}
