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#include <bits/stdc++.h>

using namespace std;

#define pb push_back
#define ll long long
#define vi vector<int>
#define vvi vector<vi>
#define vll vector<ll>
#define mp make_pair
#define fi first
#define se second
#define PII pair<int,int>
#define vPII vector<PII>
const ll MAXN = 5e5 + 5;

int n, k, a, p;
ll c;
bool seen[MAXN];

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cin >> n >> k;
    for (int i = 0; i < n && p < k; ++i) {
        cin >> a;
        if (!seen[a]) {
            seen[a] = true;
            c += (i - p);
            p++;
        }
    }
    if (p < k) cout << -1;
    else cout << c;
}