a=list(map(int,input().split())) a.sort() if 2*a[1] == a[0]+a[2]: print(a[2] + (a[2]-a[1])) elif (a[2]-a[1])*2 == (a[1]-a[0]): print(a[0] + (a[2]-a[1])) elif (a[2]-a[1]) == (a[1]-a[0])*2: print(a[1] + (a[1]-a[0]))