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#include <bits/stdc++.h>

using namespace std;

typedef long long ll;

const ll INF = 1000000000000000;

int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);

    int n;
    ll k;
    cin >> n >> k;

    int a[n + 1];
    for (int i = 1; i <= n; i++) {
        cin >> a[i];
    }

    set<int> S;
    ll res = 0;

    for (int i = 1; i <= n && S.size() < k; i++) {
        if (S.find(a[i]) == S.end()) {
            S.insert(a[i]);
            res += i;
        }
    }

    if (S.size() == k) {
        res -= k * (k + 1) / 2;
    }
    else {
        res = -1;
    }

    cout << res << "\n";

    return 0;
}