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#include <bits/stdc++.h>
#define f first
#define s second
#define mp make_pair
#define pb push_back
#define pii pair<int, int>
#define pll pair<long long, long long>
#define ll long long
#define vi vector<int>
#define vl vector<ll>
#define rep(i,p,k) for(int i=p; i<k; i++)
#define per(i,p,k) for (int i=k-1;i>=p;i--)

ll gcd(ll a,ll b) {return b?gcd(b,a%b):a;}

using namespace std;

int tab[1000005];
set<int>s;

int main() {
    int n, k;
    cin >> n >> k;
    rep(i, 0, n)
        cin >> tab[i];

    int found = 0;
    ll moves = 0;
    rep(i, 0, n) {
        if (found == k)
            break;
        if (s.find(tab[i]) == s.end()) {
            s.insert(tab[i]);
            moves += i-found;
            found++;
        }
    }
    if (found == k)
        cout << moves;
    else
        cout << -1;
}