using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using static System.Math;
using static System.Console;
using static CS_Contest.Utils;

namespace CS_Contest {
	using Li = List<int>;
	using LLi = List<List<int>>;
	class Program {
        static void Main(string[] args) {
			new Calc().Solve();
			return;
        }

		private class Calc {
			public void Solve() {
				var ABCD = ReadInt(' ');
				int A = ABCD[0], B = ABCD[1], C = ABCD[2], D = ABCD[3];
				if (A > C) {
					int t = A;
					A = C;
					C = t;
					t = B;
					B = D;
					D = t;
				}
				if (A <= C&&C<=B&&B<D) {
					(B - C).WL();
					return;
				}
				if (B < C||D<A) {
					0.WL();
					return;
				}
				if (A <= C && D <= B) {
					(D - C).WL();
				}
				return;
			}
		}
    }

	
	
    public static class Utils {
		public static int INF = (int)(1e9+7);
		public static long Mod(long x) => x % INF;
		public static long ModPow(long x,long n) { long tmp = 1; while (n != 0) { if (n % 2 == 1) { tmp = Mod(tmp * x); }x = Mod(x * x);n /= 2; }return tmp; }
		public static long DivMod(long x, long y) => Mod(x * ModPow(y, (long)(1e9 + 5)));
        public static void WL(this object obj) => WriteLine(obj);
        public static void WL(this string obj) => WriteLine(obj);
        public static void WL<T>(this IEnumerable<T> list) => list.ToList().ForEach(x => x.WL());
		public static int ReadInt() => int.Parse(ReadLine());
		public static List<int> ReadInt(char s) => ReadLine().Split(s).Select(int.Parse).ToList();
		public static long ReadLong() => long.Parse(ReadLine());
		public static List<long> ReadLong(char s) => ReadLine().Split(s).Select(long.Parse).ToList();
		public static void ReadMulti(out int x, out int y) {
			x = ReadInt();
			y = ReadInt();
		}
		public static void ReadMulti(out long x, out long y) {
			x = ReadInt();
			y = ReadInt();
		}
		public static void ReadTriple(out int x, out int y, out int z) {
			ReadMulti(out x, out y);
			z = ReadInt();
		}

		

		public static void REP(int n,Action<int> act) {
			for (int i = 0; i < n; i++) {
				act(i);
			}
		}
		public static void RREP(int n,Action<int> act) {
			for (int i = n-1; i >=0; i--) {
				act(i);
			}
		}
		public static void REP(long n, Action<int> act) => REP((int)n, act);
		public static void RREP(long n, Action<int> act) => RREP((int)n, act);

		public static void Yes() => "Yes".WL();
		public static void No() => "No".WL();
		public static void YES() => "YES".WL();
		public static void NO() => "NO".WL();


		public static int ManhattanDistance(int x1, int y1, int x2, int y2) => Abs(x2 - x1) + Abs(y2 - y1);

		public class Tree {
			public LLi Graph;
			public Tree(int nodes) {
				Graph = new LLi();
				REP(nodes, x => {
					Graph.Add(new Li());
				});
			}
			public void Add(int x,int y) {
				Graph[x].Add(y);
				Graph[y].Add(x);
			}
		}

		public class Vector2 {
			public int X { get; set; }
			public int Y { get; set; }
			public Vector2() { X = 0;Y = 0; }
			public Vector2(int x,int y) { X = x;Y = y; }
			public int ManhattanDistance(Vector2 v2) {
				return Utils.ManhattanDistance(X, Y, v2.X, v2.Y);
			}
			public static int ManhattanDistance(Vector2 v1, Vector2 v2) => v2.ManhattanDistance(v2);
			public double Distance(Vector2 v2) => Sqrt(Pow(X - v2.X, 2) + Pow(Y - v2.Y, 2));
			public static Vector2 Zero { get { return new Vector2(); } }
			public void Clear() { X = 0;Y = 0; }

		}
    }
}


Measured memory_bytes: 11220