import strutils
import sequtils
import algorithm
import math
import queues
import tables
import sets
import future
 
const INF* = high(int) div 4
 
proc readLine*(): string =
    stdin.readLine()
proc readSeq*(): seq[string] =
    readLine().strip().split()
proc readSeq*(n: Natural): seq[string] =
    result = newSeq[string](n)
    for i in 0..<n:
        result[i] = readLine().strip()
proc readInt1*(): int =
    readSeq().map(parseInt)[0]
proc readInt2*(): (int, int) =
    let a = readSeq().map(parseInt)
    return (a[0], a[1])
proc readInt3*(): (int, int, int) =
    let a = readSeq().map(parseInt)
    return (a[0], a[1], a[2])
proc readInt4*(): (int, int, int, int) =
    let a = readSeq().map(parseInt)
    return (a[0], a[1], a[2], a[3])
proc newSeqWith*[T](n: Natural; e: T): seq[T] =
    result = newSeq[T](n)
    for i in 0..<n:
        result[i] = e
type seq2*[T] = seq[seq[T]]
proc newSeq2*[T](n1, n2: Natural): seq2[T] =
    newSeqWith(n1, newSeq[T](n2))
type seq3*[T] = seq[seq[seq[T]]]
proc newSeq3*[T](n1, n2, n3: Natural): seq3[T] =
    newSeqWith(n1, newSeqWith(n2, newSeq[T](n3)))

proc join*[T](a:openArray[T],sep:string=" "):string=
    result = $a[0]
    for i in 1..a.len-1:
        result = result & " " & $a[i]

proc readIntSeq*():seq[int]=
    readline().split().map(parseInt)

##################################################################


var
    (a,b,c,d)=readInt4()


echo (b-a+1)*(d-c+1) - max(min(b,d)-max(a,c)+1,0)