a,b,c,d=list(map(int,input().split()))
hoge1,hoge2=[i for i in range(a,b+1)],[i for i in range(c,d+1)]
hoge3=list()
for e1 in hoge1:
    for e2 in hoge2:
        if e1!=e2:
            hoge3.append([e1,e2])
print(len(hoge3))