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#include <string>
#include <iostream>
#include <algorithm>
using namespace std;

int main(int argc, char const *argv[])
{
    int n, k;
    int arr[500001];
    int pos[500001];
    cin >> n >> k;
    long long result = 0;
    for (int i = 0; i < n; i++)
    {
        pos[i] = -1;
        cin >> arr[i];
    }
    pos[n] = -1;
    for (int i = k - 1; i >= 0; i--)
    {
        if (pos[arr[i]] == -1)
        {
            pos[arr[i]] = i;
        }
        else
        {
            while (k < n && pos[arr[k]] != -1)
            {
                k++;
            }
            if (k == n)
            {
                cout <<-1 << endl;
                return 0;
            }
            result += k - pos[arr[i]];
            pos[arr[k]] = pos[arr[i]];
            pos[arr[i]] = i;
        }
    }
    cout << result << endl;
    return 0;
}