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

using namespace std;

int n, k, d;
map<int, int> pos;
vector<int> v;

int main()
{
    ios::sync_with_stdio(false);
    cin >> n >> k;
    for(int i = 0; i < n; i++) {
        cin >> d;
        if(pos.find(d) == pos.end()) {
            pos[d] = i;
        } else {
            pos[d] = min(pos[d], i);
        }
    }

    if (pos.size() < k) {
        cout << -1;
        return 0;
    }
    
    map<int, int>::iterator it;

    for (it = pos.begin(); it != pos.end(); it++) {
        v.push_back(it->second);
    }
    
    sort(v.begin(), v.end());
    long long tmp = 0L;
    for(int i=0; i<k; i++) {
        tmp += (long long)(v[i] - i);
    }
    cout << tmp;

    return 0;
}