#include <bits/stdc++.h>
#define N 200005
using namespace std;
string s;
char ch;
int n,k,i,l,st,dr,mid,sol,b,e,x;
int J[N],O[N],I[N],tj,to,ti;
int mn=2e9;
int main()
{
cin>>n>>k>>s;
l=s.size();
for(i=0;i<l;i++)
{
ch=s[i];
if(ch=='J')
{
J[++tj]=i;
}
if(ch=='O')
{
O[++to]=i;
}
if(ch=='I')
{
I[++ti]=i;
}
}
for(i=1;i<=tj;i++)
{
if(i+k-1>tj) break;
b=J[i];
x=J[i+k-1]+1;
if(x>=l) continue;
st=1; dr=to; sol=-1;
while(st<=dr)
{
mid=(st+dr)>>1;
if(O[mid]<x) st=mid+1;
else {sol=mid; dr=mid-1;}
}
if(sol<0 || sol+k-1>to) continue;
x=O[sol+k-1]+1;
if(x>=l) continue;
st=1; dr=ti; sol=-1;
while(st<=dr)
{
mid=(st+dr)>>1;
if(I[mid]<x) st=mid+1;
else {sol=mid; dr=mid-1;}
}
if(sol<0 || sol+k-1>ti) continue;
e=I[sol+k-1];
mn=min(mn,(e-b+1)-3*k);
}
if(mn==2e9) cout<<-1;
else cout<<mn;
return 0;
}
