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#include <iostream>
using namespace std;
int liczby[500000];
bool tab[500000];
int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    int n, k, t = 0;
    cin >> n;
    cin >> k;
    for (int i = 0; i < n; i++)
        cin >> liczby[i];
    int temp=0;
    for (int i = 0; k > 0 and i < n;)
    {
        if (tab[liczby[i]] == 1)
        {
            t++;
            i++;
        }
        else
        {
            temp++;
            k--;
            tab[liczby[i]] = 1;
            i = temp;
        }
    }
    if (k != 0)
    {
        cout << -1;
        return 0;
    }
    else cout << t;
    return 0;
}