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#include <iostream>
using namespace std;
typedef long long ll;

bool seen[500000];

int main() {
    ios::sync_with_stdio(false); cin.tie();

    int n, k; cin >> n >> k;
    ll swaps = 0;
    int unique = 0;

    for (int i = 0; i < n && unique < k; i++) {
        int a; cin >> a; a--;
        if (!seen[a]) {
            seen[a] = true;
            swaps += i - unique;
            unique++;
        }
    }

    if (unique != k) {
        cout << "-1\n";
    } else {
        cout << swaps << '\n';
    }
}