#include<stdio.h>

int main(){
   int a,b,c,k;

   scanf("%d %d %d %d", &a, &b, &c, &k);

   if(k <= a){
      printf("%d\n", k);
   }else if(k <= a + b){
      printf("%d\n", a);
   }else{
      printf("%d\n", a-(k-a-b));
   }
   return 0;
}
