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#include <bits/stdc++.h>
#define ss second
#define ff first
#define pb push_back

//Igor Rostkowski


using namespace std;

bool po[500010];
int n,k,a,w,p;

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);

    cin>>n>>k;

    for(int i=0;i<n;i++)
    {
        cin>>a;
        if(po[a]&&i<k)
        {
            w+=abs(k-i);
            p++;
        }

        if(i>=k&&!po[a]&&p>0)
        {
            p--;
            w+=abs(k-i);
        }

        po[a]=1;
    }

    if(p>0)
    {
        cout<<"-1";
    }
    else
    {
        cout<<w;
    }
    
    return 0;
}