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();
		int t1,t2;
		if(D<=A || B<=C)System.out.print(0);
		else {
			t1=(int)Math.max(C,A);
			t2=(int)Math.min(D,B);
			System.out.print(t2-t1);
		}
		sc.close();
	}
}
/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