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#include <bits/stdc++.h>
#define pb push_back

using namespace std;
typedef long long ll;


ll cntRevs(vector<ll> &v, ll a, ll b) {
    if (a == b) return 0;
    ll m = (a + b) / 2;
    ll res = 0;
    res += cntRevs(v, a, m);
    res += cntRevs(v, m+1, b);

    vector<ll> pom;

    ll leftI = a;
    ll rightI = m+1;

    while (leftI <= m && rightI <= b) {
        if (v[leftI] > v[rightI]) {
            pom.pb(v[rightI++]);
            res += m - leftI + 1;
        }
        else pom.pb(v[leftI++]);
    }

    while (leftI <= m) pom.pb(v[leftI++]);
    while (rightI <= b) pom.pb(v[rightI++]);

    for (int i = a; i <= b; ++i) {
        v[i] = pom[i-a];
    }

    return res;
}

int main()
{
    ios_base::sync_with_stdio(0);
    ll n, k;
    cin >> n >> k;
    vector<ll> v(n), ord(n, 0);
    map<ll, bool> exist;

    ll uniq = 0;
    ll ordId = 0;
    for (int i = 0; i < n; ++i) {
        cin >> v[i];
        if (!exist[v[i]]) {
            uniq++;
            exist[v[i]] = true;
            ord[i] = ++ordId;
        }
    }

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

    for (int i = 0; i < ord.size(); ++i) {
        if (ord[i] == 0) ord[i] = ++ordId;
    }

    ll result = cntRevs(ord, 0, ord.size() - 1);
    cout << result << endl;
}