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#include <bits/stdc++.h>
using namespace std;




int main() {
    ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
    int n, k;
    cin >> n >> k;
    int cnt = 0;
    long long ans = 0;
    vector < bool > used(n + 1);
    for (int i = 0; i < n; i++) {
        int x;
        cin >> x;
        if (!used[x]) {
            ans += i - cnt;
            cnt++;
            if (cnt == k) return cout << ans << "\n", 0;
        }
        used[x] = true;
    }
    cout << "-1\n";
}