import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int a = scan.nextInt();
		int b = scan.nextInt();
		int c = scan.nextInt();
		int d = scan.nextInt();
		int[] alice = new int[101];
		int[] bob = new int[101];

		for (int i = a + 1; i <= b; i++) {
			alice[i] = 1;
		}
		for (int i = c + 1; i <= d; i++) {
			bob[i] = 1;
		}
		int ans = 0;
		for (int i = 0; i <= 100; i++) {
			if (alice[i] == 1 && bob[i] == 1) {
				ans++;
			}
		}

		System.out.println(ans);

	}

}
/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