#include <stdio.h>
int main(void){
    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",2*a-k+b);
}
