#include<stdio.h>
int main()
{
int k;
int a;
int b;
int c;
int ans;

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;
}