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