#include <bits/stdc++.h>
using namespace std;
#define en '\n'
#define sp ' '
typedef long long ll;
#define Linux ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define pii pair<int,int>
const int N=2e5+10;
const ll M=1e9+7;
int n,k;
string s;
int sj[N],so[N],si[N];
int mi=M;
int sum;
int main(){Linux
cin >> n >> k;
cin >> s;
s='0'+s;
for(int i=1;i<=n;i++){
if(s[i]=='J')sj[i]++;
else if(s[i]=='O')so[i]++;
else si[i]++;
sj[i]+=sj[i-1];
so[i]+=so[i-1];
si[i]+=si[i-1];
// cout << sj[i] << ',' << so[i] << ',' << si[i] << sp;
}
sj[n+1]=sj[n];
so[n+1]=so[n];
si[n+1]=si[n];
// cout << (lower_bound(sj,sj+n+1,3)-sj) << en;
for(int i=1;i<=n;i++){
// cout << i << sp;
sum=0;
int it1=lower_bound(sj,sj+n+1,sj[i-1]+k)-sj;
if(it1<=n){
sum+=it1-i+1-sj[it1]+sj[i-1];
// cout << it1 << sp;
int it2=lower_bound(so,so+n+1,so[it1]+k)-so;
if(it2<=n){
sum+=it2-it1-so[it2]+so[it1];
// cout << it2 << sp;
int it3=lower_bound(si,si+n+1,si[it2]+k)-si;
if(it3<=n){
sum+=it3-it2-si[it3]+si[it2];
// cout << it3 << sp;
}
else continue;
}
else continue;
}
else continue;
// cout << sum << en;
mi=min(mi,sum);
}
if(mi==M)cout << -1 << en;
else cout << mi << en;
return 0;
}
