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#include <iostream>
#include <vector>

using namespace std;

int main()
{
    int n, k;

    cin >> n >> k;

    vector<int> a(n), to_remove(n, 0);
    vector<int> taken(n + 1, 0); // 0 element unused
    for (int i = 0; i < n; i++)
        cin >> a[i];

    int taken_cnt = 0;
    int last_taken_idx = -1;
    for (int i = 0; i < n; i++) {
        if (taken[a[i]] == 0) {
            taken[a[i]] = 1;
            taken_cnt++;
        }
        else {
            to_remove[i] = 1;
        }

        if (taken_cnt == k) {
            last_taken_idx = i;
            break;
        }
    }

    if (taken_cnt < k) {
        cout << "-1" << endl;
    }
    else {
        int res = 0;
        for (int i = last_taken_idx; i >= 0; i--) {
            if (to_remove[i]) {
                res += last_taken_idx - i;
                last_taken_idx--;
            }
        }
        cout << res << endl;
    }
}