#include <algorithm>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <deque>
#include <queue>
#include <stack>
#include <vector>
#include <unordered_map>
#include <unordered_set>
#include <map>
#include <set>
using namespace std;

#define FOR(i, l, r)  for (int(i) = int(l); int(i) <= int(r); ++i)
#define FORD(i, r, l) for (int(i) = int(r); int(i) >= int(l); --i)
#define hashmap unordered_map
#define hashset unordered_set
#define fi first
#define se second
#define pb push_back
#define mp make_pair
#define sz(x) int((x).size())
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define itr iterator
#define rtr reverse_iterator

enum enum_t {White, Black, Gray};
typedef double          db;
typedef long long       ll;     typedef unsigned long long  ull;
typedef pair<int, int>  pi;     typedef pair<ll, ll>    pll;
typedef map<int, int>   mi;     typedef map<ll, ll>     mll;
typedef set<int>        si;     typedef vector<si>      vsi;
typedef set<ll>         sll;    typedef vector<sll>     vsll;
typedef set<pi>         spi;    typedef vector<spi>     vspi;
typedef set<pll>        spll;   typedef vector<spll>    vspll;
typedef vector<enum_t>  ve;     typedef vector<ve>      v2_e;
typedef vector<string>  vs;     typedef vector<vs>      v2_s;
typedef vector<bool>    vb;     typedef vector<vb>      v2_b;
typedef vector<char>    vc;     typedef vector<vc>      v2_c;
typedef vector<int>     vi;     typedef vector<vi>      v2_i;    
typedef vector<ll>      vll;    typedef vector<vll>     v2_ll;
typedef vector<pi>      vpi;    typedef vector<vpi>     v2_pi;
typedef vector<pll>     vpll;   typedef vector<vpll>    v2_pll;
typedef vector<v2_i>    v3_i;   typedef vector<v3_i>    v4_i;   
typedef vector<v2_ll>   v3_ll;  typedef vector<v3_ll>   v4_ll;   

inline int readInt()       {char c;while(c=getchar(),c!='-'&&(c<'0'||c>'9'));bool sign=(c=='-');if(sign)c=getchar();int n=c-'0';while(c=getchar(),c>='0'&&c<='9')n=10*n+c-'0';return(!sign)?n:-n;}
inline ll readLong()       {char c;while(c=getchar(),c!='-'&&(c<'0'||c>'9'));bool sign=(c=='-');if(sign)c=getchar();ll  n=c-'0';while(c=getchar(),c>='0'&&c<='9')n=10*n+c-'0';return(!sign)?n:-n;}
inline void getInt(int& n) {char c;while(c=getchar(),c!='-'&&(c<'0'||c>'9'));bool sign=(c=='-');if(sign)c=getchar();    n=c-'0';while(c=getchar(),c>='0'&&c<='9')n=10*n+c-'0';      if(sign)n=-n;}
inline void getLong(ll& n) {char c;while(c=getchar(),c!='-'&&(c<'0'||c>'9'));bool sign=(c=='-');if(sign)c=getchar();    n=c-'0';while(c=getchar(),c>='0'&&c<='9')n=10*n+c-'0';      if(sign)n=-n;}
inline string readString() {char c;while(c=getchar(),c==' '||c=='\n'||c=='\t');string s({c});while(c=getchar(),c!=EOF&&c!=' '&&c!='\n'&&c!='\t')s+=c;return s;}
char bufWrite[20];
template<typename T>inline void writeInt(T x)               {if(x<0)putchar('-'),x=-x;char*bufPos=bufWrite;do{*bufPos++=x%10+'0';}while(x/=10);do{putchar(*--bufPos);}while(bufPos>bufWrite);}
template<typename T>inline void printInt(T x, char c = EOF) {if(x<0)putchar('-'),x=-x;char*bufPos=bufWrite;do{*bufPos++=x%10+'0';}while(x/=10);do{putchar(*--bufPos);}while(bufPos>bufWrite);if(c!=EOF)putchar(c);}
template<typename T>inline T gcd(T a,T b){while(b!=0)swap(b,a%=b);return a;}
template<typename T>inline T lcm(T a,T b){return a/gcd(a,b)*b;}
template<typename T>inline tuple<T,T,T>extgcd(T a,T b){if(!a)return mt(b,0,1);T g,x,y;tie(g,x,y)=extgcd(b%a,a);return mt(g,y-(b/a)*x,x);}
template<typename T>inline bool bitGet   (T  x, int bit)         { return T(1) & (x >> bit); }
template<typename T>inline void bitSet   (T &x, int bit)         { x |=  (T(1) << bit); }
template<typename T>inline void bitReset (T &x, int bit)         { x &= ~(T(1) << bit); }
template<typename T>inline void bitFlip  (T &x, int bit)         { x ^=  (T(1) << bit); }
template<typename T>inline void bitMake  (T &x, int bit, bool v) { x ^=  (T(1) << bit) & (-v ^ x); }
/// ======================================================================
/// ======================================================================
// const db oo = 0.1 / 0; /// inf
const ll LINF = (1LL << 60); /// 1e18
const int INF = (1 << 30); /// 1e9
const int LIM = (1 << 24); /// 1e7
const int MOD = 1e9 + 7;
const int BASE = 1e4;
const int DIGIT = 4;


/// ======================================================================


int query()
{


    return 0;
}

int main()
{
    int a = readInt();
    int b = readInt();
    int c = readInt();
    int n = readInt();
    cout << min(n, a) - min(max(0, n - a - b), c) << endl;
    return 0;
}

Measured memory_bytes: 3164