#include <bits/stdc++.h>
using namespace std;
char wej[200003];
vector<int> pref[3];
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
int n,k;
cin >> n >> k;
pref[0].resize(n+1);
pref[1].resize(n+1);
pref[2].resize(n+1);
for (int i = 1; i<=n; i++){
cin >> wej[i];
pref[0][i]=pref[0][i-1];
pref[1][i]=pref[1][i-1];
pref[2][i]=pref[2][i-1];
if (wej[i]=='J')pref[0][i]++;
else if (wej[i]=='O')pref[1][i]++;
else pref[2][i]++;
}
int ans=n+1;
for (int i = 1; i<=n+1-3*k; i++){
int p0=i,p1=lower_bound(pref[0].begin()+p0,pref[0].end(),pref[0][i-1]+k)-pref[0].begin();
if (p1>n)continue;
int p2=lower_bound(pref[1].begin()+p1,pref[1].end(),pref[1][p1-1]+k)-pref[1].begin();
if (p2>n)continue;
int p3=lower_bound(pref[2].begin()+p2,pref[2].end(),pref[2][p2-1]+k)-pref[2].begin();
if (p3>n)continue;
ans=min(ans,p3-p0+1-3*k);
}
if (ans>n)cout << "-1\n";
else cout << ans << '\n';
}
