#include <iostream>
#include <vector>
#include <unordered_map>

using namespace std;

int n, k;
vector<int> as;
unordered_map<int, int> map;
vector<int> first_order;

int main()
{
    cin >> n >> k;
    as.resize(n);
    for (int i = 0; i < n; i++)
    {
        cin >> as[i];
    }

    for (int i = 0; i < n; i++)
    {
        if(map.find(as[i]) == map.end()){
            map[as[i]] = i;
            first_order.push_back(as[i]);
        }
    }

    if(map.size() < k){
        cout << "-1\n";
        return 0;
    }
    
    int result = 0;
    for (int i = 0; i < k; i++)
    {
        result += map[first_order[i]] - i;
    }
    
    cout << result << '\n';
}