#include    <iostream>
#include    <fstream>

using namespace std;

ifstream fin("fisier.in");

int main()
{
    int nr, nrPrec;
    int ratCrt, ratPrec, ratLungMax;
    int lung = 0, lungMax = 0;

    fin >> nrPrec;
    while(fin >> nr)
    {
        ratCrt = nr - nrPrec;
        if(ratPrec == ratCrt)
        {
            lung++;
            if(lung > lungMax)
            {
                lungMax = lung;
                ratLungMax = ratCrt;
            }
        }
        else
            lung = 0;

        ratPrec = ratCrt;
        nrPrec = nr;
    }

    if(lungMax != 0)
        cout << ratLungMax;
    else
        cout << "nu exista";
    return 0;
}