#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int main(){

    int n, k;
    long result = 0;

    cin >> n >> k;

    vector<int> tab(n);
    vector<int> found;

    for(int i = 0; i < n; i++)
    {
        cin >> tab[i];
    }

    found.push_back(tab[0]);

    for(int i = 1; i < n; i++)
    {
        if(std::find(found.begin(), found.end(), tab[i]) != found.end()) {
            /* found contains element */
            continue;
        } else {
            /* vector found does not contain it yet - new uniique element */
            found.push_back(tab[i]);
            result += i+1 - found.size();
            //cout << i << " " << tab[i] << " " << found.size() << endl;
            if(found.size() == k) break;
        }
    }

if(found.size() == k)
    {
        cout << result;
    }
    else 
    {
        cout << -1;
    }

    return 0;
}