import java.util.Scanner;

public class Main {

	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

		int A, B, C, D;
		int start, end;
		int ans;

		A = Integer.parseInt(sc.next());
		B = Integer.parseInt(sc.next());
		C = Integer.parseInt(sc.next());
		D = Integer.parseInt(sc.next());

		if(B < C || D < A){
			ans = 0;
		}else{
			start = Math.max(A, C);
			end = Math.min(B, D);
			ans = end - start;
		}

		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