n = list(map(int, input().split()))
all = (n[1] - n[0] + 1)*(n[3] - n[2] + 1)
if(n[1] > n[2]):
    excp = n[1] - n[2] + 1
else:
    excp = n[2] - n[1] + 1
print(all - excp)