#include <bits/stdc++.h>
using namespace std;

const int maxN = 5e5+5;
int a[maxN];

bitset<maxN> butelka_id;

void solve()
{
    long long ans = 0;
    int n, k, next_butelka = 1, pos = 1;
    cin >> n >> k;
    for(int i = 1; i <= n; i++)
        cin >> a[i];

    while(next_butelka <= k && pos <= n) {
        if(butelka_id[a[pos]] == 1) {
            pos++;
            continue;
        }

        ans += (pos - next_butelka);

        butelka_id[a[pos]] = 1;
        next_butelka++;
        pos++;
    }

    if (next_butelka <= k) {
        cout << -1;
    }
    else {
        cout << ans;
    }
}

int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);

    solve();

    return 0;
}