import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt(),b = sc.nextInt(),c = sc.nextInt(),d = sc.nextInt();
		if(b<c || a>d) {
			System.out.println(0);
		}else if(c<=a) {
			System.out.println(Math.min(b,d)-a);
		}else if(b<=d) {
			System.out.println(b-Math.max(a, c));
		}else if(c<=b) {
			System.out.println(Math.min(b,d)-c);
		}else if(a<=d) {
			System.out.println(d-Math.max(a, c));
		}
	}
	public static int sample() {
		return 1;
	}

}

/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