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#include <bits/stdc++.h>

using namespace std;

int main() {
    int n, k;
    cin >> n >> k;
    vector<int> a(n);

    for (int i=0; i<n; ++i)
        cin >> a[i];

    vector<bool> seen(n+1, false);

    int cnt = 0;
    int i = 0;
    int steps = 0;

    while (i < n and cnt < k) {
        if (not seen[a[i]]) {
            seen[a[i]] = true;
            steps += i - cnt;
            cnt++;
        }
        ++i;
    }

    if (cnt == k)
        cout << steps;
    else
        cout << -1;

    return 0;
}