import sys

lines = {}
for i , val in enumerate(sys.stdin):
    num = list(map(float,val.split()))
    lines[f"x{i+1}"] = num[0]
    lines[f"y{i+1}"] = num[1]

    if i >=1:
        break

x1,y1,x2,y2 = lines.values()
de = ((x1-x2)**2 + (y1-y2)**2)**0.5
dm = abs(x1-x2) + abs(y1-y2)

print(abs(dm -de))