#include<bits/stdc++.h>
#include<string.h>
#include <algorithm>
#include <stdlib.h>
#define ll long long
using namespace std;
ll k[5][100000],a,b,c,d,e,f,m,i,j,n,h,g,l,r,ka,p,q,t[200005];
map<ll,ll> maa,mii,mee;
vector<ll> vj,vo,vi;
string x,y,z;
int main(){
cin>>a>>b;
cin>>x;
for(i=0 ; i<a ; i++){
if(x[i]=='J'){
vj.push_back(i);
}
if(x[i]=='O'){
vo.push_back(i);
}
if(x[i]=='I'){
vi.push_back(i);
}
}
if(vj.size()<b){
cout<<-1;
return 0;
}
if(vo.size()<b){
cout<<-1;
return 0;
}
if(vi.size()<b){
cout<<-1;
return 0;
}
h=-1;
for(i=0 ; i<vo.size()-b+1 ; i++){
if(vo[i]>vj[b-1]){
h=i;
break;
}
}
if(h==-1){
cout<<h;
return 0;
}
if(vo[h+b-1]>vi[vi.size()-b]){
cout<<-1;
return 0;
}
h=100000000000;
for(i=0 ; i<vj.size()-b+1 ; i++){
n=vj[i+b-1];
l=0;
r=vo.size()-b;
if(vo[r]<n){
break;
}
while(l<r){
m=(l+r-1)/2;
if(vo[m]>n){
r=m;
}
else{
l=m+1;
}
}
n=vo[l+b-1];
l=0;
r=vi.size()-b;
if(vi[r]<n){
break;
}
while(l<r){
m=(l+r-1)/2;
if(vi[m]>n){
r=m;
}
else{
l=m+1;
}
}
l+=b-1;
h=min(h,(a-3*b-vj[i]-(a-vi[l]-1)));
}
cout<<h<<endl;
}
