A, B, C, D = map(int,input().split())
if B < C or D < A:
    print((B-A+1)*(D-C+1))
elif C <= B and B <= D:
    print((B-A+1)*(D-C+1)-(B-C+1))
elif C <= B and D <= B:
    print((B-A+1)*(D-C+1)-(D-C+1))
elif C <= A and B <= D:
    print((B-A+1)*(D-C+1)-(B-A+1))
elif C <= A and A <= D:
    print((B-A+1)*(D-C+1)-(D-A+1))