observe.cpp

#include <iostream>

void merge(int arr[], int l, int m, int r)
{
    int n1 = m - l + 1;
    int n2 = r - m;

    int L[n1], R[n2];

    for (int i = 0; i < n1; i++)
        L[i] = arr[l + i];
    for (int j = 0; j < n2; j++)
        R[j] = arr[m + 1 + j];

    int i = 0, j = 0, k = l;

    while (i < n1 && j < n2)
    {
        if (L[i] <= R[j])
        {
            arr[k] = L[i];
            i++;
        }
        else
        {
            arr[k] = R[j];
            j++;
        }
        k++;
    }

    while (i < n1)
    {
        arr[k] = L[i];
        i++;
        k++;
    }

    while (j < n2)
    {
        arr[k] = R[j];
        j++;
        k++;
    }
}

void mergeSort(int arr[], int l, int r)
{
    if (l < r)
    {
        int m = l + (r - l) / 2;

        mergeSort(arr, l, m);
        mergeSort(arr, m + 1, r);

        merge(arr, l, m, r);
    }
}

int main()
{
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(NULL);

    int N;
    std::cin >> N;
    int arr[N];
    for (int i = 0; i < N; i++)
        std::cin >> arr[i];

    // sort the array
    mergeSort(arr, 0, N - 1);

    int diff = arr[1] - arr[0];
    for (int i = 2; i < N; i++)
    {
        if (arr[i] - arr[i - 1] != diff)
        {
            std::cout << "NO" << std::endl;
            return 0;
        }
    }

    std::cout << "YES" << std::endl;
    return 0;
}