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#include <iostream>

using namespace std;

int main()
{
    int n;
    cin >> n;
    int a[n];
    for (int i = 0; i < n; i++)
    {
        cin >> a[i];
    }

    int max = 0;
    int rest;
    int result;

    for (int i = 0; i < n; i++)
    {
        if (a[i] > max)
            max = a[i];
    }

    int m = max + 20;
    int count[m];
    for (int i = 0; i < m; i++)
    {
        count[i] = 0;
    }

    for (int i = 0; i < n; i++)
    {
        count[a[i]] +=1;
    }

    for (int i = 0; i < m-1; i++)
    {
        if (count[i] != 0)
        {
            rest = count[i]/2;
            count[i] = count[i]%2;
            count[i+1] = count[i+1] + rest;
        }
    }

    for (int i = m-1; i >= 0; i--)
    {
        if (count[i] == 1)
        {
            result = i;
            break;
        }
    }

    cout << result << endl;

return 0;
}