import java.util.Scanner;

class Main {

	public static void main(String args[]){

		Scanner scan = new Scanner(System.in);
		int a, b, c, d;
		a = scan.nextInt();
		b = scan.nextInt();
		c = scan.nextInt();
		d = scan.nextInt();
		
		if(a <= c && c <= b)
			if(b <= d)System.out.print(b-c);
			else System.out.print(d-c);
		else if(c <= a && a <= d)
			if(d <= b)System.out.print(d-a);
			else System.out.print(b-a);
		else System.out.print(0);
		
	}
}
/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