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#include <bits/stdc++.h>

int main()
{
    using namespace std;
    int n, k;
    cin >> n >> k;
    vector<int> brand(n);
    for (int i = 0; i < n; ++i)
    {
        cin >> brand[i];
    }

    unordered_set<int> seen;
    seen.insert(brand[0]);
    int to_skip = 0;
    int unique = 1;
    long long swaps = 0;
    for (int i = 1; i < n; ++i)
    {
        if (seen.count(brand[i]))
        {
            ++to_skip;
        }
        else
        {
            swaps += to_skip;
            ++unique;
            if (unique == k) break;
        }
    }

    cout << (unique == k ? swaps : -1) << endl;

    return 0;
}