import array
seq=[input(),input(),input(),input()]
a=int(seq[0])
b=int(seq[1])
c=int(seq[2])
d=int(seq[3])
if (a+c-2*b)==0:
print('此為等差數列,')
print('此數列第五項為:'+str(2*d-c))
aW1wb3J0IGFycmF5CnNlcT1baW5wdXQoKSxpbnB1dCgpLGlucHV0KCksaW5wdXQoKV0KYT1pbnQoc2VxWzBdKQpiPWludChzZXFbMV0pCmM9aW50KHNlcVsyXSkKZD1pbnQoc2VxWzNdKQppZiAoYStjLTIqYik9PTA6CiAgICBwcmludCgn5q2k54K6562J5beu5pW45YiX77yMJykKICAgIHByaW50KCfmraTmlbjliJfnrKzkupTpoIXngrrvvJonK3N0cigyKmQtYykp