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#include <iostream>
using namespace std;

int main(){
    
    int n;
    cin >> n;
    int tab[n];
    for(int i = 0; i < n; i++)
        cin >> tab[i];
    
    const int leng = 201718 + 1;
    int aux[leng];
    for(int j = 0; j < leng; j++)
        aux[j] = 0;
    
    int h = 0;
    for(int k = 0; k < n; k++)
    {
        h = tab[k];
        aux[h] += 1;
    }
    int end = leng - 1;
    bool b = 1;
    for(int p = leng - 1; (p >= 0) && b; p--)
    {
        if(aux[p] != 0)
        {
            b = 0;
            end = p;
        }
    }
    if(b == 1)
        end = 0;
        
    bool stop2 = 0;
    int index = 0;
    int res = 0;
    bool stop = 1;
    while(stop)
    {
        res = res / 2;
        if(index < end)
            res += aux[index];
        else if(index == end)
        {
          res += aux[index];
          if(res <= 1)
            stop2 = 1;
        }
        else
            stop2 = 1;
        if(stop2 && (res <= 1))
            stop = 0;
        else
            index++;
    }
    
    cout << index;
    
    return 0;
}