import java.util.*;
class Main
{
  public static void main(String args[])
  {
    Scanner sc=new Scanner(System.in);
    int a=sc.nextInt();
    int b=sc.nextInt();
    int c=sc.nextInt();
    int k=sc.nextInt();
    int sum=0;
    for(int i=0;i<k;i++)
    {
      if(a!=0)
      {
        sum++;
        a--;
      }
      else 
      {
        if(b!=0)
        {
          b--;
        }
        else
        {
          c--;
          sum--;
        }
      }
    }
    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