import java.util.*;

class Main{
	public static void main(String[] args){
    	Scanner sc = new Scanner(System.in);
      int A = sc.nextInt();  //  1
      int B = sc.nextInt();  //  0
      int C = sc.nextInt();  // -1
      int K = sc.nextInt();  // 選ぶ枚数
      int max = -100;
      
      if ( A >= K){
      	System.out.println(K);
      }
      else{
        int L = K - A;
     	 if ( B >= L){
        	System.out.println(A);
      	}
      	else
      	{
      		System.out.println(1*A + 0*B+ (L-B)*(-1) );
      	}
      }
    }
}
/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