import java.util.*;
public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner sc = new Scanner(System.in);
		
		int arr[] = new int[103];
		
		int a = sc.nextInt(), b = sc.nextInt(), c = sc.nextInt(), d = sc.nextInt();
		arr[a+1]++; arr[b+1]--; arr[c+1]++; arr[d+1]--;
		
		
		int count = 0;
		for(int i = 1; i<=100; i++) {
			arr[i] += arr[i-1];
			if(arr[i] == 2) count++;
		}
		
		System.out.println(count);
	}

}

/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