#sdpsgssolutions
#Missing Mango
#AIO 2017 (Intermediate)

# Open the input and output files.
inputFile = open("manin.txt", "r")
outputFile = open("manout.txt", "w")

# Read how many numbers there will be
myInput = inputFile.readline().split()
Ix = int(myInput[0])
Cx = int(myInput[1])
Id = int(myInput[2])
Cd = int(myInput[3])

# Possible Positions around Ishraq
position1 = Ix + Id
position2 = Ix - Id

# Possible Positions around Clement
position3 = Cx + Cd
position4 = Cx - Cd

# Finalise Positions
if position1 == position3:
	position = position1
elif position1 == position4:
	position = position1
elif position2 == position3:
	position = position2
else:
	position = position2

outputFile.write(str(position))

# Finally, close the input/output files.
inputFile.close()
outputFile.close()