#include<bits/stdc++.h>
#define pb push_back
#define mp make_pair
#define inf 1000000007
#define fr first
#define sc second
#define eps 1e-9
#define clr(a) memset(a, 0, sizeof(a))
#define sz(x) x.size()
#define sni(x) scanf("%d",&x)
#define snl(x) scanf("%lld", &x)
#define snc(x) scanf("%c", &c);
#define rep(n) for(int i = 0 ; i < n ;i ++)
#define repc(i, n) for(int i = 0 ; i < n ; i ++)
#define FOR(i, x, y) for(int i = x ; i < y ; i ++)
#define DEC(i, x, y) for(int i = x ; i >= y ; i --)
#define all(v) v.begin(), v.end()
#define min3(a, b, c) min(a, min(b,c))
#define max3(a, b, c) max(a, max(b, c))
#define alla(a, n) a, a+n
using namespace std;
double pi=3.1415926535897932384626433;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll,ll> ii;
typedef vector<ll> vi;
typedef vector<ii> vii;
ll MOD=1000000007;
ll C[2005][2005];
ll min(ll a,ll b)
{
 return a<b?a:b;
}

 
ll pw(ll a,ll b){
    if(b==0) return 1;
    ll x=pw(a,b/2);
    x=(x*x)%MOD;
    if(b%2) x=(x*a)%MOD;
    return x%MOD;
}
ll comb2(ll n,ll k, ll x)
{
ll res = 1LL; 
  
   
    if ( k > n - k ) 
        k = n - k; 
  
   
    for (int i = 0; i < k; ++i) 
    { 
        res = (res%MOD*(n - i)%MOD)%MOD; 
        res =(res%MOD* pw((i + 1),MOD-2)%MOD)%MOD; 
    } 
  
    //return res; 
return res%MOD;
}
void comb(ll n, ll k) 
{ 
    //int C[n+1][k+1]; 
    ll i, j; 
  
    // Caculate value of Binomial Coefficient in bottom up manner 
    for (i = 0; i <= n; i++) 
    { 
        for (j = 0; j <= min(i, k); j++) 
        { 
            // Base Cases 
            if (j == 0 || j == i) 
                C[i][j] = 1LL; 
  
            // Calculate value using previosly stored values 
            else
                C[i][j] = (C[i-1][j-1]%MOD + C[i-1][j]%MOD)%MOD; 
        } 
    } 
  
   // return C[n][k]; 
} 
ll phi(ll n) 
{ 
    ll result = n; // Initialize result as n 
  
    // Consider all prime factors of n and subtract their 
    // multiples from result 
    for (ll p = 2; p * p <= n; ++p) { 
          
        // Check if p is a prime factor. 
        if (n % p == 0) { 
              
            // If yes, then update n and result 
            while (n % p == 0) 
                n /= p; 
            result -= result / p; 
        } 
    } 
  
    // If n has a prime factor greater than sqrt(n) 
    // (There can be at-most one such prime factor) 
    if (n > 1) 
        result -= result / n; 
    return result; 
} 
ll cc(ll n) 
{ 
    // Modulus operator are expensive 
    // on most of the computers. 
    // n & 3 will be equivalent to n % 4 
    // n % 4 
    switch (n & 3) { 
  
    // If n is a multiple of 4 
    case 0: 
        return n; 
  
    // If n % 4 gives remainder 1 
    case 1: 
        return 1; 
  
    // If n % 4 gives remainder 2 
    case 2: 
        return n + 1; 
  
    // If n % 4 gives remainder 3 
    case 3: 
        return 0; 
    } 
} 
ll pp(string s)
{
    reverse(s.begin(),s.end());
    int res=0;
    for(int i=0;i<s.length();i++)
    res+=(s[i]-'0')*pow(10,i);
    return res;
}
bool check(ll aa[],ll bb[],ll n)
{
    for(ll i=0;i<n;i++)
    if(aa[i]!=bb[i])
    return true;
    
    return false;
    
}
void solve()
{
//comb(2001,2001);
std::ios::sync_with_stdio(0);
    cin.tie(NULL);
    cout.tie(NULL);
ll t;
t=1;
//cin>>t;


while(t--)
{
ll a,b,c,k;
cin>>a>>b>>c>>k;
ll s=0;
ll cc=0;
if(k-a>=0)
{
s+=(a);
k=k-a;
}
else
{
s+=(k);
k=0;
}
if(k-b>=0)
{
s+=0;
k-=b;
}
else
{
s+=0;
k=0;
}
if(k-c>=0)
{
    s-=(c);
    k=k-c;
}
else
s-=k;
cout<<s;
}
}




int main(){
    std::ios::sync_with_stdio(0);
    cin.tie(NULL);
    cout.tie(NULL);
    
    solve();
    
}












Measured memory_bytes: 3672