#include <stdio.h>

int main(int argc, char const *argv[])
{
    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;
}