#include <stdio.h>
#include <string.h>
int main()
{
    int a,b,c,k;
    scanf("%d %d %d %d",&a,&b,&c,&k);

if(k<=a){
    printf("%d",k);
}
else if(k<=a+b){
    printf("%d",a);
}
else{
    printf("%d",a-(k-a-b));
}

    

  
    return 0;
}

