using System;

namespace AtCoder.Problems
{
    public class ProblemB
    {
        public static void Main(string[] args)
        {
            var s = new SolveB();
            s.Solve();
        }

        public class Scanner
        {
            private string[] _s;
            private int _i;
            private readonly char[] _separators = new char[] { ' ' };

            public Scanner()
            {
                _s = new string[0];
                _i = 0;
            }

            public string Next()
            {
                if (_i < _s.Length) return _s[_i++];
                string st = Console.ReadLine();
                while (st == "") st = Console.ReadLine();
                _s = st.Split(_separators, StringSplitOptions.RemoveEmptyEntries);
                if (_s.Length == 0) return Next();
                _i = 0;
                return _s[_i++];
            }

            public int NextInt() => int.Parse(Next());
            public long NextLong() => long.Parse(Next());
            public ulong NextULong() => ulong.Parse(Next());
            public double NextDouble() => double.Parse(Next());

            public int[] GetIntArray(int n, int add = 0)
            {
                int[] array = new int[n];
                for (int i = 0; i < n; i++)
                {
                    array[i] = NextInt() + add;
                }
                return array;
            }

            public long[] GetLongArray(int n, long add = 0)
            {
                long[] array = new long[n];
                for (int i = 0; i < n; i++)
                {
                    array[i] = NextLong() + add;
                }
                return array;
            }

            public double[] GetDoubleArray(int n, double add = 0)
            {
                double[] array = new double[n];
                for (int i = 0; i < n; i++)
                {
                    array[i] = NextDouble() + add;
                }
                return array;
            }

            public string[] GetStringArray(int n)
            {
                string[] array = new string[n];
                for (int i = 0; i < n; i++)
                {
                    array[i] = Next();
                }
                return array;
            }
        }
    }

    public class SolveB
    {
        public void Solve()
        {
            var cin = new ProblemB.Scanner();
            var a = cin.NextLong();
            var b = cin.NextLong();
            var c = cin.NextLong();
            var k = cin.NextLong();

            var result = GetAnswer(a, b, c, k);
            Console.WriteLine(result);
        }

        public long GetAnswer(long a, long b, long c, long k)
        {
            var ans = 0L;

            {
                var numA = Math.Min(a, k);
                ans += numA;
                k -= numA;
            }

            if (k > 0)
            {
                var numB = Math.Min(b, k);
                k -= numB;
            }

            if (k > 0)
            {
                var numC = Math.Min(c, k);
                ans -= numC;
            }

            return ans;
        }
    }
}


Measured memory_bytes: 27496