#include <bits/stdc++.h>
using namespace std;
int main(){
int n,k;
cin>>n>>k;
string s;
cin>>s;
s = 'K'+s;
vector<int> jpos,ipos;
int oct[n]; oct[0] = 0;
int mpj[n], mpi[n];
memset(mpj,-1,sizeof(mpj)); memset(mpi,-1,sizeof(mpi));
for(int i=1; i<=n; i++){
if(s[i]=='J'){ jpos.push_back(i); mpj[i] = jpos.size()-1;}
if(s[i]=='I'){ ipos.push_back(i); mpi[i] = ipos.size()-1;}
if(s[i]=='O') oct[i]=oct[i-1]+1;
else oct[i]=oct[i-1];
}
int min_span = 1e9;
int jsz=jpos.size(),isz=ipos.size();
int j_r = jpos[jsz-1], j_l = jpos[jsz-k];
int i_l=-1,i_r=-1;
for(int i=j_r+1; i<=n; i++){
if(oct[i-1]-oct[j_r]>=k && s[i]=='I'){
i_l = i; break;
}
}
if(i_l != -1){
if(mpi[i_l] + k <= isz) i_r = ipos[mpi[i_l]+k-1];
}
if(i_r!=-1) min_span = min(min_span, i_r-j_l+1);
//cout<<min_span<<endl;
for(int i=jsz-k-1; i>=0; i--){
j_l = jpos[i]; j_r = jpos[i+k-1];
//cout<<j_l<<' '<<j_r<<endl;
if(i_l==-1){
if(oct[ipos[isz-1]-1] - oct[j_r] >= k) i_l=ipos[isz-1];
else continue;
}
int ival=0;
for(int v=i_l; v>0; v--){
if(oct[v-1] - oct[j_r] < k){ ival=v; break;}
}
//binse(ival+1);
int lll=0, rrr=isz-1;
while(lll<rrr){
int mmm = (lll+rrr)/2;
if(ipos[mmm] > ival) rrr=mmm;
else lll=mmm+1;
}
i_l = ipos[lll];
if(i_l != -1){
if(mpi[i_l] + k <= isz) i_r = ipos[mpi[i_l]+k-1];
}
//cout<<i_l<<' '<<i_r<<endl;
//cout<<mpi[i_l]<<endl;
if(i_r!=-1) min_span = min(min_span, i_r-j_l+1);
//cout<<min_span<<endl;
}
//cout<<min_span<<endl;
if(min_span==1e9) cout<<-1;
else cout<<(min_span-(3*k));
}
