px, py = map(int, input().split())
qx, qy = map(int, input().split())
time = 0
x_diff = abs(px-qx)
y_diff = abs(py-qy)

if x_diff >= 1:
  time += x_diff/2
# elif not x_diff == 0:
#   time += 1

if y_diff >= 1:
  time += y_diff/2
# elif not y_diff == 0:
#   time += 1
if int(time) == time:
  time = int(time)
  
print(time)