#include <bits/stdc++.h>
#define fst first
#define snd second
#define pb push_back
#define forn(i,a,b) for(int i= a; i<b; i++)
#define ALL(x) x.begin(),x.end()
#define SZ(x) (int)x.size()
using namespace std;
typedef long long ll;
#ifdef D
#define debug(x) cout << x
#else
#define debug(x) //nothing
#endif
#define INF ((ll)1000000000)
int main(){
ll n, k; cin>>n>>k;
string s; cin>>s;
vector<ll> rs(n);
forn(i,0,n){
if(s[i]=='J') rs[i]=0;
else if(s[i]=='O') rs[i]=1;
else rs[i]=2;
}
vector<vector<ll>> his(3);
debug("precalculo listo\n");
ll res=-1;
for(int i = n-1; i>=0; i--){
his[rs[i]].pb(i);
}
forn(i,0,3) sort(ALL(his[i]));
forn(i,0,n){
ll last = i;
forn(j,0,3){
ll ind = lower_bound(ALL(his[j]),last)-his[j].begin();
ind+=k-1;
debug("toca en "<<j<<" "<<last<<" "<<ind<<'\n');
if(ind>=SZ(his[j])) last=INF;
else last=his[j][ind];
}
debug("para "<<i<<" "<<last<<'\n');
if(last!=INF){
if(res==-1 || res>((last+1)-i)-3*k){
res=((last+1)-i)-3*k;
}
}
}
cout<<res<<'\n';
return 0;
}
