import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

a,b,c,d = map(int,read().split())

total = (b - a + 1) * (d - c + 1)

overlap = min(b,d) - max(a,c) + 1
x = total - overlap if overlap > 0 else total
print(x)