#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
#define Rep(i,a,n)for(int i=(int)(a);i<(int)(n);i++)
#define rep(i,n)Rep(i,0,n)
typedef long long ll;
typedef long double ld;
#define Ma(x,y) ((x)>(y)?(x):(y))
#define mi(x,y) ((x)<(y)?(x):(y))

int main() {
	int a, b, c, k, ans = 0;
	scanf("%d %d %d %d", &a, &b, &c, &k);
	if (k < a)ans = k;
	else if (k <= a + b)ans = a;
	else ans = a + a + b - k;
	printf("%d", ans);
}