#include <stdio.h>
#include <stdlib.h>

// Comparator function for qsort
int compare(const void *a, const void *b) {
    return ((int)a-(int)b);
}

int main() {
    int n;
    scanf("%d", &n);
    int arr[n];
    
    for (int i = 0; i < n; i++) {
        scanf("%d", &arr[i]);
    }
    
    // Sort the array to handle pairs in sorted order
    qsort(arr, n, sizeof(int), compare);
    
    int found = 0;
    printf("[");
    for (int i = 0; i < n; i++) {
        // To avoid duplicates, skip if current is same as previous
        if (i > 0 && arr[i] == arr[i-1]) continue;
        for (int j = i + 1; j < n; j++) {
            // To avoid duplicates in pairs
            if (j > i + 1 && arr[j] == arr[j-1]) continue;
            if (arr[i] + arr[j] == 0) {
                if (found) printf(", ");
                printf("[%d, %d]", arr[i], arr[j]);
                found = 1;
                break; // Move to next i after finding pair
            }
        }
    }
    printf("]\n");
    
    if (!found) {
        printf("[]\n");
    }
    
    return 0;
}