import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int c = sc.nextInt();
        int d = sc.nextInt();
        sc.close();

        int sec;
        if (a <= c && c <= b && d <= b) {
            sec = d - c;
        } else if (a <= c && c <= b && d >= b) {
            sec = b - c;
        } else if (a >= c && a <= d && d <= b) {
            sec = d - a;
        } else if (a >= c && a <= d && d > b) {
            sec = b - a;
        } else {
            sec = 0;
        }
        System.out.println(sec);
    }
}

/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