n = int(input()) s = list(map(int,input().split())) def is_f(m): for i in range(2,len(m)): if m[i]-m[i-1] != m[i-1]-m[i-2]: return False return True if is_f(s): print(s[-1]+(n-len(s))*(s[-1]-s[-2])) else: print(s[-1]*(s[-1]//s[-2])**(n-len(s)))