#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define pii pair<int,int>
#define ff first
#define ss second
#define pb push_back
#define vi vector<int>
#define fr(i,ii,iii) for(int i=ii;i<iii;i++)
const int N=2e5+5;
int n,k;
int c[N][3];
map<char,int>mp;
string a;
int find(int x,int d){
int l=x,r=n+1,m,ans=-1;
while(l<r){
m=(l+r)/2;
int cn=c[m][d]-c[x-1][d];
if(cn>=k){
ans=m;
r=m;
}
else{
l=m+1;
}
}
return ans;
}
int main(){
mp['J']=0;
mp['O']=1;
mp['I']=2;
cin>>n>>k>>a;
a='@'+a;
int ans=1e6;
fr(i,1,n+1){
fr(j,0,3)c[i][j]=c[i-1][j];
c[i][mp[a[i]]]++;
}
fr(i,1,n+1){
int i1=i;
fr(j,0,3){
i1=find(i1,j);
if(i1==-1)goto end;
}
ans=min(ans,i1-i+1-3*k);
}
end:;
cout<<(ans==1e6?-1:ans);
}
