A, B, C, D = map(int,input().split())
ans = (B-A+1)*(D-C+1)
m = min(A, B, C, D)
M = max(A, B, C, D)
kabri = 0
for i in range(m,M+1):
    if A <= i and i <= B and C <= i and i <= D:
        kabri += 1
print(ans-kabri)