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#include <bits/stdc++.h>
using namespace std;

bool present[500007];
bool drink[500007];
int t[500007];

int n, k;
int different;
long long swaps;

int main() {

    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);

    cin >> n >> k;

    for (int i = 0; i < n; i++) {
        cin >> t[i];
        different += !present[t[i]];
        present[t[i]] = true;
    }

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

    for (long long i = 0LL, pointer = 0LL; i < k; i++, pointer++) {
        if (drink[t[i]]) {
            while (drink[t[pointer]]) {
                pointer++;
            }
            swap(t[i], t[pointer]);
            swaps += pointer - i;
        }
        drink[t[i]] = true;
    }

    cout << swaps;
    return 0;
}