a,b,c,d = map(int,input().split())
red = [i for i in range(a,b+1)]
blk = [i for i in range(c,d+1)]
array=[]

for i in red:
    tmp=[i]
    for j in blk:
        tmp.append(j)
        if not tmp in array and tmp[0]!=tmp[1]:
            array.append(tmp)
        tmp=[i]
            
print(len(array))