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#include <bits/stdc++.h>
using namespace std;
int n,k;
int a[500005];
int l[500005];
vector <int> pos;


int main ()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    cin>> n>> k;
    for (int i=1; i<=n; i++)
        cin>> a[i];
    for (int i=n; i>=1; i--)
        l[a[i]]=i;

    for (int i=1; i<=n; i++)
        if (l[i])
            pos.push_back(l[i]-1);
    sort (pos.begin(), pos.end());

    if (pos.size()<k)
    {
        cout<< -1;
        return 0;
    }

    long long ans=0;
    for (int i=0; i<k; i++)
        if (pos[i]>i)
            ans+=(long long)(pos[i]-i);

    cout<< ans;


    return 0;
}