

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.StringTokenizer;

public class Main {

	static long expo(long base, long exp) {
		long t = 1L;
		while (exp > 0) {
			if (exp % 2 != 0)
				t = (t * base);

			base = (base * base);
			exp /= 2;
		}
		return t;
	}

	public static void main(String args[]) throws Exception {
		MyScanner sc = new MyScanner();
		PrintWriter pr = new PrintWriter(System.out);
		long a = sc.nextLong(), b = sc.nextLong(), c = sc.nextLong(), k = sc.nextLong();
		long sum = 0;
		
		if(k > a)
			sum += a;
		else
			sum += k;
		
		k -= a;
		k -= b;
		if (k > 0)
			sum -= k;

		pr.println(sum);
		pr.close();
	}

	public static class MyScanner {
		BufferedReader br;
		StringTokenizer st;

		public MyScanner() {
			br = new BufferedReader(new InputStreamReader(System.in));
		}

		String next() {
			while (st == null || !st.hasMoreElements()) {
				try {
					st = new StringTokenizer(br.readLine());
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
			return st.nextToken();
		}

		int nextInt() {
			return Integer.parseInt(next());
		}

		long nextLong() {
			return Long.parseLong(next());
		}

		double nextDouble() {
			return Double.parseDouble(next());
		}

		String nextLine() {
			String str = "";
			try {
				str = br.readLine();
			} catch (IOException e) {
				e.printStackTrace();
			}
			return str;
		}
	}
} import java.io.BufferedReader; BufferedReader java.io import java.io.IOException; IOException java.io import java.io.InputStreamReader; InputStreamReader java.io import java.io.PrintWriter; PrintWriter java.io import java.util.StringTokenizer; StringTokenizer java.util public class Main {

	static long expo(long base, long exp) {
		long t = 1L;
		while (exp > 0) {
			if (exp % 2 != 0)
				t = (t * base);

			base = (base * base);
			exp /= 2;
		}
		return t;
	}

	public static void main(String args[]) throws Exception {
		MyScanner sc = new MyScanner();
		PrintWriter pr = new PrintWriter(System.out);
		long a = sc.nextLong(), b = sc.nextLong(), c = sc.nextLong(), k = sc.nextLong();
		long sum = 0;
		
		if(k > a)
			sum += a;
		else
			sum += k;
		
		k -= a;
		k -= b;
		if (k > 0)
			sum -= k;

		pr.println(sum);
		pr.close();
	}

	public static class MyScanner {
		BufferedReader br;
		StringTokenizer st;

		public MyScanner() {
			br = new BufferedReader(new InputStreamReader(System.in));
		}

		String next() {
			while (st == null || !st.hasMoreElements()) {
				try {
					st = new StringTokenizer(br.readLine());
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
			return st.nextToken();
		}

		int nextInt() {
			return Integer.parseInt(next());
		}

		long nextLong() {
			return Long.parseLong(next());
		}

		double nextDouble() {
			return Double.parseDouble(next());
		}

		String nextLine() {
			String str = "";
			try {
				str = br.readLine();
			} catch (IOException e) {
				e.printStackTrace();
			}
			return str;
		}
	}
} Main static long expo(long base, long exp) {
		long t = 1L;
		while (exp > 0) {
			if (exp % 2 != 0)
				t = (t * base);

			base = (base * base);
			exp /= 2;
		}
		return t;
	} expo {
		long t = 1L;
		while (exp > 0) {
			if (exp % 2 != 0)
				t = (t * base);

			base = (base * base);
			exp /= 2;
		}
		return t;
	} long t = 1L; t 1L while (exp > 0) {
			if (exp % 2 != 0)
				t = (t * base);

			base = (base * base);
			exp /= 2;
		} exp > 0 exp 0 {
			if (exp % 2 != 0)
				t = (t * base);

			base = (base * base);
			exp /= 2;
		} if (exp % 2 != 0)
				t = (t * base); exp % 2 != 0 exp % 2 exp 2 0 t = (t * base) t (t * base) t base base = (base * base) base (base * base) base base exp /= 2 exp 2 return t; t long base base long exp exp public static void main(String args[]) throws Exception {
		MyScanner sc = new MyScanner();
		PrintWriter pr = new PrintWriter(System.out);
		long a = sc.nextLong(), b = sc.nextLong(), c = sc.nextLong(), k = sc.nextLong();
		long sum = 0;
		
		if(k > a)
			sum += a;
		else
			sum += k;
		
		k -= a;
		k -= b;
		if (k > 0)
			sum -= k;

		pr.println(sum);
		pr.close();
	} main {
		MyScanner sc = new MyScanner();
		PrintWriter pr = new PrintWriter(System.out);
		long a = sc.nextLong(), b = sc.nextLong(), c = sc.nextLong(), k = sc.nextLong();
		long sum = 0;
		
		if(k > a)
			sum += a;
		else
			sum += k;
		
		k -= a;
		k -= b;
		if (k > 0)
			sum -= k;

		pr.println(sum);
		pr.close();
	} MyScanner sc = new MyScanner(); sc new MyScanner() PrintWriter pr = new PrintWriter(System.out); pr new PrintWriter(System.out) long a = sc.nextLong() a sc.nextLong() sc.nextLong sc b = sc.nextLong() b sc.nextLong() sc.nextLong sc c = sc.nextLong() c sc.nextLong() sc.nextLong sc k = sc.nextLong(); k sc.nextLong() sc.nextLong sc long sum = 0; sum 0 if(k > a)
			sum += a;
		else
			sum += k; k > a k a sum += a sum a sum += k sum k k -= a k a k -= b k b if (k > 0)
			sum -= k; k > 0 k 0 sum -= k sum k pr.println(sum) pr.println pr sum pr.close() pr.close pr String args[] args public static class MyScanner {
		BufferedReader br;
		StringTokenizer st;

		public MyScanner() {
			br = new BufferedReader(new InputStreamReader(System.in));
		}

		String next() {
			while (st == null || !st.hasMoreElements()) {
				try {
					st = new StringTokenizer(br.readLine());
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
			return st.nextToken();
		}

		int nextInt() {
			return Integer.parseInt(next());
		}

		long nextLong() {
			return Long.parseLong(next());
		}

		double nextDouble() {
			return Double.parseDouble(next());
		}

		String nextLine() {
			String str = "";
			try {
				str = br.readLine();
			} catch (IOException e) {
				e.printStackTrace();
			}
			return str;
		}
	} MyScanner BufferedReader br; br StringTokenizer st; st public MyScanner() {
			br = new BufferedReader(new InputStreamReader(System.in));
		} MyScanner {
			br = new BufferedReader(new InputStreamReader(System.in));
		} br = new BufferedReader(new InputStreamReader(System.in)) br new BufferedReader(new InputStreamReader(System.in)) String next() {
			while (st == null || !st.hasMoreElements()) {
				try {
					st = new StringTokenizer(br.readLine());
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
			return st.nextToken();
		} next {
			while (st == null || !st.hasMoreElements()) {
				try {
					st = new StringTokenizer(br.readLine());
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
			return st.nextToken();
		} while (st == null || !st.hasMoreElements()) {
				try {
					st = new StringTokenizer(br.readLine());
				} catch (IOException e) {
					e.printStackTrace();
				}
			} st == null || !st.hasMoreElements() st == null st null !st.hasMoreElements() st.hasMoreElements() st.hasMoreElements st {
				try {
					st = new StringTokenizer(br.readLine());
				} catch (IOException e) {
					e.printStackTrace();
				}
			} try {
					st = new StringTokenizer(br.readLine());
				} catch (IOException e) {
					e.printStackTrace();
				} catch (IOException e) {
					e.printStackTrace();
				} IOException e {
					e.printStackTrace();
				} e.printStackTrace() e.printStackTrace e {
					st = new StringTokenizer(br.readLine());
				} st = new StringTokenizer(br.readLine()) st new StringTokenizer(br.readLine()) return st.nextToken(); st.nextToken() st.nextToken st int nextInt() {
			return Integer.parseInt(next());
		} nextInt {
			return Integer.parseInt(next());
		} return Integer.parseInt(next()); Integer.parseInt(next()) Integer.parseInt Integer next() next long nextLong() {
			return Long.parseLong(next());
		} nextLong {
			return Long.parseLong(next());
		} return Long.parseLong(next()); Long.parseLong(next()) Long.parseLong Long next() next double nextDouble() {
			return Double.parseDouble(next());
		} nextDouble {
			return Double.parseDouble(next());
		} return Double.parseDouble(next()); Double.parseDouble(next()) Double.parseDouble Double next() next String nextLine() {
			String str = "";
			try {
				str = br.readLine();
			} catch (IOException e) {
				e.printStackTrace();
			}
			return str;
		} nextLine {
			String str = "";
			try {
				str = br.readLine();
			} catch (IOException e) {
				e.printStackTrace();
			}
			return str;
		} String str = ""; str "" try {
				str = br.readLine();
			} catch (IOException e) {
				e.printStackTrace();
			} catch (IOException e) {
				e.printStackTrace();
			} IOException e {
				e.printStackTrace();
			} e.printStackTrace() e.printStackTrace e {
				str = br.readLine();
			} str = br.readLine() str br.readLine() br.readLine br return str; str public class Main {

	static long expo(long base, long exp) {
		long t = 1L;
		while (exp > 0) {
			if (exp % 2 != 0)
				t = (t * base);

			base = (base * base);
			exp /= 2;
		}
		return t;
	}

	public static void main(String args[]) throws Exception {
		MyScanner sc = new MyScanner();
		PrintWriter pr = new PrintWriter(System.out);
		long a = sc.nextLong(), b = sc.nextLong(), c = sc.nextLong(), k = sc.nextLong();
		long sum = 0;
		
		if(k > a)
			sum += a;
		else
			sum += k;
		
		k -= a;
		k -= b;
		if (k > 0)
			sum -= k;

		pr.println(sum);
		pr.close();
	}

	public static class MyScanner {
		BufferedReader br;
		StringTokenizer st;

		public MyScanner() {
			br = new BufferedReader(new InputStreamReader(System.in));
		}

		String next() {
			while (st == null || !st.hasMoreElements()) {
				try {
					st = new StringTokenizer(br.readLine());
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
			return st.nextToken();
		}

		int nextInt() {
			return Integer.parseInt(next());
		}

		long nextLong() {
			return Long.parseLong(next());
		}

		double nextDouble() {
			return Double.parseDouble(next());
		}

		String nextLine() {
			String str = "";
			try {
				str = br.readLine();
			} catch (IOException e) {
				e.printStackTrace();
			}
			return str;
		}
	}
} public class Main {

	static long expo(long base, long exp) {
		long t = 1L;
		while (exp > 0) {
			if (exp % 2 != 0)
				t = (t * base);

			base = (base * base);
			exp /= 2;
		}
		return t;
	}

	public static void main(String args[]) throws Exception {
		MyScanner sc = new MyScanner();
		PrintWriter pr = new PrintWriter(System.out);
		long a = sc.nextLong(), b = sc.nextLong(), c = sc.nextLong(), k = sc.nextLong();
		long sum = 0;
		
		if(k > a)
			sum += a;
		else
			sum += k;
		
		k -= a;
		k -= b;
		if (k > 0)
			sum -= k;

		pr.println(sum);
		pr.close();
	}

	public static class MyScanner {
		BufferedReader br;
		StringTokenizer st;

		public MyScanner() {
			br = new BufferedReader(new InputStreamReader(System.in));
		}

		String next() {
			while (st == null || !st.hasMoreElements()) {
				try {
					st = new StringTokenizer(br.readLine());
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
			return st.nextToken();
		}

		int nextInt() {
			return Integer.parseInt(next());
		}

		long nextLong() {
			return Long.parseLong(next());
		}

		double nextDouble() {
			return Double.parseDouble(next());
		}

		String nextLine() {
			String str = "";
			try {
				str = br.readLine();
			} catch (IOException e) {
				e.printStackTrace();
			}
			return str;
		}
	}
} Main


import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.StringTokenizer;

public class Main {

	static long expo(long base, long exp) {
		long t = 1L;
		while (exp > 0) {
			if (exp % 2 != 0)
				t = (t * base);

			base = (base * base);
			exp /= 2;
		}
		return t;
	}

	public static void main(String args[]) throws Exception {
		MyScanner sc = new MyScanner();
		PrintWriter pr = new PrintWriter(System.out);
		long a = sc.nextLong(), b = sc.nextLong(), c = sc.nextLong(), k = sc.nextLong();
		long sum = 0;
		
		if(k > a)
			sum += a;
		else
			sum += k;
		
		k -= a;
		k -= b;
		if (k > 0)
			sum -= k;

		pr.println(sum);
		pr.close();
	}

	public static class MyScanner {
		BufferedReader br;
		StringTokenizer st;

		public MyScanner() {
			br = new BufferedReader(new InputStreamReader(System.in));
		}

		String next() {
			while (st == null || !st.hasMoreElements()) {
				try {
					st = new StringTokenizer(br.readLine());
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
			return st.nextToken();
		}

		int nextInt() {
			return Integer.parseInt(next());
		}

		long nextLong() {
			return Long.parseLong(next());
		}

		double nextDouble() {
			return Double.parseDouble(next());
		}

		String nextLine() {
			String str = "";
			try {
				str = br.readLine();
			} catch (IOException e) {
				e.printStackTrace();
			}
			return str;
		}
	}
}
