import java.util.Scanner;
 
public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
	int num1 = Integer.parseInt(sc.next());
	int num0 = Integer.parseInt(sc.next());
    int numMinus1 = Integer.parseInt(sc.next());
    int numToSelect = Integer.parseInt(sc.next());

    int sum = 0;
	if(numToSelect <= num1){
    	sum = numToSelect * 1;
    }else if(num1 < numToSelect && numToSelect <= (num1+num0)){
        sum = num1 * 1;
    }else if((num1+num0) < numToSelect && numToSelect <= (num1+num0+numMinus1)){
        sum = num1 * 1 + (numToSelect-(num1+num0)) * -1;
    }else if((num1+num0+numMinus1) < numToSelect){
      	sum = num1 * 1 + numMinus1 * -1;
    }

    System.out.println(sum);
  }
}
/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