/////////// <----------------------  Quicksilver_ ---------------------> ///////////
#include <bits/stdc++.h>
#include <sstream>
#include <algorithm>
#include <map>
#include <iterator>
#include <queue>
#include <utility>
#include <set>

using namespace std;
#define endl "\n"
#define ll long long int
#define c(x) ll x;cin>>x
#define cc(x,y) ll x,y;cin>>x>>y
#define ccc(x,y,z) ll x,y,z; cin>>x>>y>>z
#define db long double
#define fast cin.tie(NULL); cout.tie(NULL); ios_base::sync_with_stdio(false)
#define inf 200000000000000
#define mod 1000000007
#define eps 1e-7
#define PI 3.1415926535897932385
#define pb push_back
#define bitc  __builtin_popcountll
#define mp make_pair
#define ff first
#define ss second
#define all(ar) ar.begin(), ar.end()
#define len(x) (ll)(x).size()
#define fr(i,a,b) for (ll i = (a), _b = (b); i <= _b; i++)
#define rep(i,n) for (ll i = 0, _n = (n); i < _n; i++)
#define repr(i,n) for (ll i = n - 1; i >= 0; i--)
#define frr(i,a,b) for (ll i = (a), _b = (b); i >= _b; i--)
#define foreach(it,ar) for (auto it = ar.begin(); it != ar.end(); it++)
#define fill(ar,val) memset(ar, val, sizeof(ar))
#define print(arr,n) cout<<" :: "<<#arr<<" :: "<<endl; rep(i,n) cout<<arr[i]<<" "; cout<<endl
#define print2(arr,n,m) cout<<" :: "<<#arr<<" :: "<<endl; rep(i,n){ rep(j,m) cout<<arr[i][j]<<" "; cout<<endl; }
#define dyn1(arr,n) ll* arr=new ll[n]; rep(i,n) arr[i]=0;
#define dyn2(arr,n,m) ll** arr=new ll*[(n)]; rep(i,n) arr[i]=new ll[m]; rep(i,n) rep(j,m) arr[i][j]=0
#define carr(arr,n) rep(i,n) cin>>arr[i]
#define carr2(arr,n,m) rep(i,n) rep(j,m) cin>>arr[i][j]
#define debug(a) cout<<" :: "<<#a<<" == "<<a<<endl
#define debug2(a,b) cout<<" :: "<<#a<<" == "<<a<<" :: "<<#b<<" == "<<b<<endl
#define sort(a) sort(a.begin(), a.end())
typedef pair<ll,ll> pii;
typedef vector<pii> vii;

inline ll maxim(ll a,ll b) {if(a>b) return a; else return b;}
inline ll minim(ll a,ll b) {if(a<b) return a; else return b;}
inline bool equals(double a, double b){ return fabs(a - b) < 1e-9; }
ll gcd(ll a, ll b){ return b==0 ? a : gcd(b, a%b); }


bool prime[10002];
void SieveOfEratosthenes()
{
    memset(prime, true, sizeof(prime));
    prime[1]=false;
    for (int p=2; p*p<=10001; p++)
    {
        if (prime[p] == true)
        {
            for (int i=p*p; i<=10001; i += p)
                prime[i] = false;
        }
    }
}

ll binpow(ll a, ll b)
{
    ll res = 1;
    while (b > 0)
    {
        if (b & 1) res = (res * a);
        a = (a * a);
        b >>= 1;
    }
    return res;
}

ll modpow(ll a, ll b)
{
    ll res = 1;
    while (b > 0)
    {
        if (b & 1) res = (res * a)%mod;
        a = (a * a)%mod;
        b >>= 1;
    }
    return res;
}

int modInverse(int a, int m)
{
    int m0 = m;
    int y = 0, x = 1;
    if (m == 1)
      return 0;
    while (a > 1)
    {
        // q is quotient
        int q = a / m;
        int t = m;
        // m is remainder now, process same as
        // Euclid's algo
        m = a % m, a = t;
        t = y;
        // Update y and x
        y = x - q * y;
        x = t;
    }
    // Make x positive
    if (x < 0)
       x += m0;
    return x;
}

ll func(ll h)
{
  return (h*(3*h+1))/2;
}


ll bs(ll low,ll high,ll target)
{
  while(low+3<high)
  {
    ll mid=(low+high)/2;
    if(func(mid)<=target)
    {
      low=mid;
    }
    else
    {
      high=mid-1;
    }
  }


  frr(o,high,low)
  {
    if(func(o)<=target)
    {
      return o;
    }
  }
  return -1;
}


ll arr[1000010];
ll visited[1000010]={0};
ll suff[1000010];
ll colouring[1000010]={0};
vector<ll> adj[1000010];


void adj_former(ll n)
{
  rep(o,n-1)
  {
    cc(a,b);
    adj[a].pb(b);
    adj[b].pb(a);
  }
}


void rec(ll node,ll colour)
{
  colouring[node]=colour;
  visited[node]=1;
  rep(i,adj[node].size())
  {
    if(!visited[adj[node][i]])
    {
      rec(adj[node][i],1-colour);
    }
  }
}


int main()
{
  fast;

  #ifndef ONLINE_JUDGE
  // for getting input from input.txt
  freopen("input.txt", "r", stdin);
  // for writing output to output.txt
  freopen("output.txt", "w", stdout);
	#endif

  cout<<setprecision(10)<<fixed;
  ll a,b,c,k;
  cin>>a>>b>>c>>k;
  ll rem=k;
  ll total=0;
  total+=minim(rem,a);
  rem=maxim(rem-a,0);
  rem=maxim(rem-b,0);
  total-=rem;
  cout<<total<<endl;

  return 0;
}


Measured memory_bytes: 27088