import java.util.*;
public class Main {
	public static void main(String[] args){
	Scanner sc = new Scanner(System.in);
	int A = sc.nextInt();
	int B = sc.nextInt();
	int C = sc.nextInt();
	int D = sc.nextInt();
	if(B - C <= 0 || D - A <= 0){
		System.out.println(0);
	}else{
		System.out.println(Math.min(B,D) - Math.max(A,C));
	}
	}
}

/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