#include <iostream>
#include <unordered_map>
using namespace std;

void solve()
{
    int value;
    int n, k;
    cin >> n >> k;
    int last = 0;
    int result = 0;
    bool done = false;
    unordered_map<int, int> m;
    for (int i = 0; i < n; i++) {
        cin >> value;
        // if (m.contains(value)) {
        if (m.find(value) != m.end()) {
            continue;
        } else {
            m[value] = 0;
            result += i - last;
            last++;
        }

        if (last == k) {
            done = true;
            break;
        }
    }

    if (done) {
        cout << result << endl;
    } else {
        cout << -1 << endl;
    }
}

int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie();
    solve();
    return 0;
}