#include<bits/stdc++.h>
using namespace std;
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;
#define int long long
#define FOR(i,a) for(int i=0;i<(a);i++)
#define FORE(i,a,b) for(int i=(a);i<(b);i++)
#define sp <<' '<<
#define fi first
#define se second
#define pb push_back
typedef vector<int> vi;
typedef pair<int,int> pii;
typedef vector<pii> vii;
typedef vector<bool> vb;
typedef tree<int,null_type,less<int>,rb_tree_tag,tree_order_statistics_node_update> oset;
#define all(x) x.begin(),x.end()
#define cont(x) for(auto el:x) cout<<el<<' ';cout<<endl;
#define contp(x) for(auto el:x) cout<<el.fi<<'-'<<el.se<<' ';cout<<endl;
#define DEBUG(x) cout<<#x sp x<<endl;
#define carp(a,b) ((a%MOD)*(b%MOD))%MOD
#define topla(a,b) ((a%MOD)+(b%MOD))%MOD
#define mid (l+r)/2
const int MAXN=305;
const int MAXK=25;
const int MOD=1e9+7;
const int INF=1e18;
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(0);cout.tie(0);
//freopen("out.txt","w",stdout);
int n,k;
cin>>n>>k;
string s;
cin>>s;
deque<int> j,o,ii;
FOR(i,n){
if(s[i]=='J'){
j.pb(i);
}
else if(s[i]=='O'){
o.pb(i);
}
else ii.pb(i);
}
int ans=-1;
int bas,son;
set<pii> tut, tut2;
while(j.size()>=k){
bas=j.front();
if(j.size()<k) break;
son=j[k-1];
j.pop_front();
int cost=son-bas+1-k;
tut.insert({son,cost});
}
while(!tut.empty() and o.size()>=k){
bas=o.front();
if(o.size()<k) break;
son=o[k-1];
o.pop_front();
int cost=son-bas+1-k;
pii p={bas,0LL};
auto it=tut.lower_bound(p);
if(it!=tut.begin()){
it--;
int ind=it->fi;
int deg=it->se;
cost=cost+deg+(bas-ind-1);
tut2.insert({son,cost});
}
}
swap(tut,tut2);
tut2.clear();
while(!tut.empty() and ii.size()>=k){
bas=ii.front();
if(ii.size()<k) break;
son=ii[k-1];
ii.pop_front();
int cost=son-bas+1-k;
pii p={bas,0LL};
auto it=tut.lower_bound(p);
if(it!=tut.begin()){
it--;
int ind=it->fi;
int deg=it->se;
cost=cost+deg+(bas-ind-1);
tut2.insert({son,cost});
}
}
if(tut2.empty()) cout<<-1<<endl;
else cout<<(tut2.begin()->se)<<endl;
}
