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#include <bits/stdc++.h>

#define DEBUG(x) cout << '>' << #x << ':' << x << endl;
#define fi first
#define se second
#define ll long long
#define ld long double
#define pb push_back
#define vi vector<int>
#define MAXN 100001

using namespace std;

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

    int n, k;
    cin >> n >> k;

    vector<bool> S(k, false);
    vector<int> V(k, -1);

    int a;
    int j = 0;
    for (int i = 0; i < n; i++) {
        cin >> a;

        if (!S[a]) {
            S[a] = true;
            V[j] = i;
            j++;
        }
    }

    ll res = 0;
    if (j < k) {
        res = -1;
    } else {
        for (int i = 0; i < k; i++) {
            res += V[i] - i;
        }
    }

    cout << res << endl;

    return 0;
}