J - Make Equal Under Mod

You have an array $A$ consisting of $N$ integers. You can do the following operation as many times as you want: - Choose $i$ and $X$ satisfying $1 \le i \le N$ and $1 \le X \le 10^7$, and change $A_i$ to $A_i \bmod X$ It can be proven that it is possible to make all elements of $A$ equal. Let $f(A)$ denote the maximum possible value of $A_1$ after all the elements of $A$ have been made equal.