#include <bits/stdc++.h>
#define int long long
#define pb push_back
using namespace std;
const int N=2e5+10;
const int inf=1e18;
const int mod=1e9+7;
vector<int>j,o,i;
signed main() {
ios_base::sync_with_stdio(NULL);
cin.tie(NULL);
int T=1;
// cin>>T;
while(T--) {
int n,k;
cin>>n>>k;
string s;
cin>>s;
for(int it=0;it<n;it++) {
if(s[it]=='J') j.pb(it);
if(s[it]=='O') o.pb(it);
if(s[it]=='I') i.pb(it);
}
int ans=inf;
for(int it=0;it<=j.size()-k;it++) {
int cur=j[it];
int p=upper_bound(o.begin(),o.end(),j[it+k-1])-o.begin()+k-1;
if(p>o.size()-1) continue;
p=upper_bound(i.begin(),i.end(),o[p])-i.begin()+k-1;
if(p>i.size()-1) continue;
// cout<<it<<' ';
cur+=n-1-i[p];
ans=min(ans,cur);
}
if(ans==inf) cout<<"-1\n";
else cout<<ans<<' ';
}
}
