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#include<bits/stdc++.h>
using namespace std;
constexpr int MAXN = 5e5 + 7;
int n, k;

int first_of_type[MAXN];
vector<int> v;

int main()
{
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    cin >> n >> k;
    for (int i = 1; i <= n; i++)
    {
        int a;
        cin >> a;
        if (first_of_type[a] == 0){
            first_of_type[a] = i;
            v.push_back(i);
        }
    }
    sort(v.begin(), v.end());
    if (v.size() < k){
        cout << "-1\n";
        return 0;
    }
    long long res = 0;
    for (int i = 0; i < k; i++)
        res += (long long) v[i];
    res -= k * (k + 1) / 2;
    cout << res << "\n";
    
    return 0;
}