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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);

    const int MAX_N = 500010;

    int n, k;
    cin >> n >> k;
    bitset<MAX_N> included;

    int cnt = 0, nextPos = 0;
    long long ans = 0;
    for(int i = 0; i < n && cnt < k; i++) {
        int type;
        cin >> type;

        if(included[type])
            continue;

        included[type] = 1;
        ans += i - nextPos++;
        cnt++;
    }

    cout << (cnt == k ? ans : -1) << "\n";

    return 0;
}