/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */


import java.io.*;
import java.util.*;

/**
 *
 * @author nai
 */
public class Main
{
    public static void main(String[] args)
    {
        StringBuilder sb = new StringBuilder();
        FastScanner sc = new FastScanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();
        int C = sc.nextInt();
        int D = sc.nextInt();
        
        int ans = Math.min(D, B) - Math.max(A, C);
        if(ans < 0)
            ans = 0;
        System.out.println(ans);
    }
    static class FastScanner {

        private BufferedReader reader = null;
        private StringTokenizer tokenizer = null;
        
        public FastScanner(InputStream in) {
            reader = new BufferedReader(new InputStreamReader(in));
            tokenizer = null;
        }
        
        public String next() {
            if (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }
        /*public String nextChar(){
            return (char)next()[0];
        }*/
        public String nextLine() {
            if (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    return reader.readLine();
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            
            return tokenizer.nextToken("\n");
        }
        
        public long nextLong() {
            return Long.parseLong(next());
        }
        
        public int nextInt() {
            return Integer.parseInt(next());
        }
        
        public double nextDouble() {
            return Double.parseDouble(next());
        }
        
        public int[] nextIntArray(int n) {
            int[] a = new int[n];
            for (int i = 0; i < n; i++) {
                a[i] = nextInt();
            }
            return a;
        }
        public char[] nextCharArray(int n){
            char[] a = next().toCharArray();
            
            return a;
        }
        public char[][] nextCharArray2(int h , int w){
            char[][] a = new char[h][w];
            for (int i = 0; i < h; i++) {
                a[i] = next().toCharArray();
            }
            return a;
        }
        public char[][] nextWrapCharArray2(int h , int w){
            char[][] a = new char[h + 2][w + 2];
            
            for (int i = 1; i < h + 1; i++) {
                a[i] = (" " + next() + " ").toCharArray();
            }
            return a;
        }
        
        
        public int [][] nextIntArray2(int h, int w){
            int[][] a = new int[h][w];
            for(int hi = 0 ; hi < h ; hi++){
                for(int wi = 0 ; wi < h ; wi++){
                    a[h][w] = nextInt();
                }
            }
            return a;
        }
        
        public long[] nextLongArray(int n) {
            long[] a = new long[n];
            for (int i = 0; i < n; i++) {
                a[i] = nextLong();
            }
            return a;
        }
        public long [][] nextLongArray2(int h, int w){
            long[][] a = new long[h][w];
            for(int hi = 0 ; hi < h ; hi++){
                for(int wi = 0 ; wi < h ; wi++){
                    a[h][w] = nextLong();
                }
            }
            return a;
        }
    }
}


Measured memory_bytes: 21204