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#include <bits/stdc++.h>

using namespace std;

void solve() {
    int n, k;
    cin >> n >> k;
    vector<int> v(n);
    for (int i = 0; i < n; i++) {
        cin >> v[i];
    }
    vector<int> used(n+1);
    long long ans = 0;
    int curr = 0;
    for (int i = 0; i < n; i++) {
        if (!used[v[i]]) {
            used[v[i]]++;
            ans += i - curr;
            curr++;
            if (curr == k) {
                cout << ans << endl;
                return;
            }
        }
    }
    cout << -1 << endl;
}

int main() {
    ios_base::sync_with_stdio(false);
    cout.precision(12);
    cout << fixed;
    cin.tie(0);

    int T = 1;
    //cin >> T;
    for (int t = 0; t < T; t++) {
        solve();
    }

    return 0;
}