import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc=new Scanner(System.in);
        String[] line;
        String result="";
        line=sc.nextLine().split(" ");
        int a=Integer.parseInt(line[0]);
        int b=Integer.parseInt(line[1]);
        int c=Integer.parseInt(line[2]);
        int d=Integer.parseInt(line[3]);
        int puttern=(b-a+1)*(d-c+1);
        int r=b;
        if(d<r) r=d;
        int l=a;
        if(l<c) l=c;
        int equals=r-l+1;
        if(equals<0) equals=0;
        result+=puttern-equals;
        System.out.println(result);
    }
}