#include<bits/stdc++.h>
#define ll long long
#define f first
#define s second
using namespace std;
const int maxn=2e5+5;
int n,k;
string s;
int J[maxn],O[maxn],I[maxn];
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);cout.tie(0);
cin>>n>>k>>s;
s=" "+s;
for(int i=1;i<=n;i++){
J[i]=J[i-1]+(s[i]=='J');
O[i]=O[i-1]+(s[i]=='O');
I[i]=I[i-1]+(s[i]=='I');
}
int ans=1e9;
for(int i=1;i<=n;i++){
int pos1=lower_bound(J+1,J+n+1,J[i-1]+k)-J;
if(J[pos1]-J[i-1]<k) continue;
int pos2=lower_bound(O+1,O+n+1,O[pos1]+k)-O;
if(O[pos2]-O[pos1]<k) continue;
int pos3=lower_bound(I+1,I+n+1,I[pos2]+k)-I;
if(I[pos3]-I[pos2]<k) continue;
ans=min(ans,(pos3-i+1)-3*k);
}
if(ans==1e9) ans=-1;
cout<<ans;
}
