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#include <bits/stdc++.h>
using namespace std;
const int MAXN = 500005;

int n, k, distinct;
long long res;
int a[MAXN], cnt[MAXN];
int32_t main() {
    ios_base::sync_with_stdio(0);
    cin >> n >> k;
    for (int i = 1; i <= n; i++) {
        cin >> a[i];
        cnt[a[i]]++;
        if (cnt[a[i]] == 1)
            distinct++;
    }

    if (distinct < k) {
        cout << "-1\n";
        return 0;
    }

    for (int i = 1; i <= n; i++) cnt[i] = 0;

    int j = 1;
    for (int i = 1; i <= k; i++) {
        if (cnt[a[i]]) {
            while(cnt[a[j]]) j++;
            res += j - i;
            cnt[a[j]]++;
        }

        cnt[a[i]]++;
    }

    cout << res << "\n";
}