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#include <iostream>
#include <vector>
#include <set>
#include <map>

using namespace std;

typedef vector<int> vint;
typedef set<int> sint;
typedef map<int, int> mint;
typedef long long ll;



int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    int n, k;
    cin >> n >> k;
    vint a;
    int ai;
    for (int i = 0; i < n; i++) {
        cin >> ai;
        a.push_back(ai);
    }
    vint to_change;
    mint k_map, seen;
    sint a_set;
    ll sum = 0, last = -1;
    bool skip = false;
    for (int i = 0; i < n; i++) {
        if (!skip) {
            if (i < k)
                k_map[a[i]] = true;
            if (i >= k && k_map[a[i]] != true) {
                if (to_change.size() == 0)
                    skip = true;
                sum += i - to_change.back();
                to_change.pop_back();
                k_map[a[i]] = true;
            }
            if (i < k && seen[a[i]] == true) {
                to_change.push_back(i);
            } else
                seen[a[i]] = true;
            last = a[i];
        }
        a_set.insert(a[i]);
    }
    if (a_set.size() < k)
        cout << -1;
    else
        cout << sum;
    return 0; 
}