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#include <iostream>

using namespace std;

int main()
{
    int n, k, bottle;
    int lastBottle = 0;
    int differentBottles = 0;
    int steps = 0;
    int result = 0;

    cin >> n >> k;

    for (int i = 0; i < n; ++i)
    {
        cin >> bottle;

        if (bottle != lastBottle)
        {
            result += steps;
            ++differentBottles;
        }
        else
        {
            ++steps;
        }

        lastBottle = bottle;
    }

    if (differentBottles < k)
    {
        cout << -1 << endl;
        return 0;
    }

    cout << result << endl;
    return 0;
}