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#include <iostream>
#include <vector>
#include <set>

using namespace std;

const int MAX_N = 5 * 100000;

int t[MAX_N];
int t_tmp[MAX_N];

long long inv(int start, int end) {

    if(end <= start) {
        return 0;
    }

    if(end == start + 1) {
        if(t[end] < t[start]) {
            int tmp = t[end];
            t[end] = t[start];
            t[start] = tmp;
            return 1;
        } else {
            return 0;
        }
    }

    int mid = (end + start) / 2;

    long long a = inv(start, mid);
    long long b = inv(mid+1, end);

    for(int i=start; i<=end; i++) {
        t_tmp[i] = t[i];
    }

    int s1 = start, s2 = mid+1;

    long long x = 0;
    int idx = start;
    while(s1 <= mid && s2 <= end) {
        if(t_tmp[s1] > t_tmp[s2]) {
            x += mid - s1 + 1;
            t[idx++] = t_tmp[s2++];
        } else {
            t[idx++] = t_tmp[s1++];
        }
    }

    while(s1 <= mid) {
        t[idx++] = t_tmp[s1++];
    }

    while(s2 <= end) {
        t[idx++] = t_tmp[s2++];
    }

    return a + b + x;

}

int main() {

    ios_base::sync_with_stdio(0);
    cin.tie(0);

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



    set<int> dist;
    for (int i = 0; i < n; i++) {
        cin >> t[i];
        dist.insert(t[i]);
    }

    if (dist.size() < k) {
        cout << "-1" << endl;
    } else {
        dist.clear();
        int next = k;
        for (int i = 0; i < n; i++) {
            if (dist.size() < k && dist.find(t[i]) == dist.end()) {
                dist.insert(t[i]);
                t[i] = dist.size() - 1;
            } else {
                t[i] = next++;
            }
        }

        long long res = inv(0, n-1);

        cout << res << endl;

    }


}