import java.util.*;
public class Test10 {
  public static void main(String[] args){
    Scanner sc=new Scanner(System.in);
    String[] line;
    line=sc.nextLine().split(" ");
    int a=Integer.parseInt(line[0]);
    int a1=Integer.parseInt(line[1]);
    int b=Integer.parseInt(line[2]);
    int b1=Integer.parseInt(line[3]);
    int eq=0,tmp,tmp1;
    tmp=a;
    if(tmp<b) tmp=b;
    tmp1=a1;
    if(b1<tmp1) tmp1=b1;
    eq=tmp1-tmp+1;
    if(eq<0) eq=0;
    int combi=(a1-a+1)*(b1-b+1)-eq;
    System.out.println(combi);
  }
}