#include <iostream>
#include <set>
#include <vector>
using namespace std;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int n,k;
    cin >> n >> k;
    vector<int>Brands(n);
    for(int &b : Brands)
        cin >> b;

    bool success = false;
    long long result = 0;
    set<int>BrandsTaken;
    for (int i = 0; i < n; i++)
        if (!BrandsTaken.count(Brands[i])) {
            result += i - BrandsTaken.size();
            BrandsTaken.insert(Brands[i]);
            if (BrandsTaken.size() == k) {
                success = true;
                break;
            }
        }
    
    cout << (success ? result : -1) << endl;
    return 0;
}