import java.util.Scanner;

public class Main{
    public static void main(String[] args){
        Scanner s=new Scanner(System.in);
        int a=s.nextInt();
        int b=s.nextInt();
        int c=s.nextInt();
        int d=s.nextInt();
        int[] x=new int[100000];
        int i,count=0;
        for(i=0;i<100000;i++)x[i]=0;
        for(i=a;i<b;i++)x[i]++;
        for(i=c;i<d;i++)x[i]++;
        for(i=0;i<100000;i++){
            if(x[i]==2)count++;
        }
        System.out.println(count);
    }
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz