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#include <iostream>
#include <list>
#include <set>
#include <bitset>

#define MAX 500001
using namespace std;

typedef long long ll;

int main() {
    ios_base::sync_with_stdio(false);

    ll a, n, k;
    cin >> n >> k;
    list<ll> nums;
    set<ll> unique;

    bitset<MAX> seen;

    while (n-- > 0) {
        cin >> a;
        unique.insert(a);
        nums.push_back(a);
    }

    if (unique.size() < k) {
        cout << -1;
        return 0;
    }

    ll pos = 0;
    ll dist = 0;
    ll sum = 0;
    auto head = nums.begin();
    auto tail = nums.begin();

    while (pos < k) {
        if (seen.test(*head)) {
            // we need to insert something here ;)
            while (seen.test(*tail)) {
                tail++;
                dist++;
            }
            head = nums.insert(head, *tail);
            tail = nums.erase(tail);
            sum += dist;
            dist++;
        } else {
            seen.set(*head, true);
            head++;
            pos++;
            dist--;
        }
    }
    cout << sum;

    return 0;
}