#include<bits/stdc++.h>
using namespace std;
namespace variables{
    typedef long double ld;
    typedef long long ll;
    typedef unsigned long long ull;
    typedef unsigned int uint;
    typedef pair<ll,ll> pll;
    struct triple{
        ll first,second,third;
    };
}
using namespace variables;
void debug(set<ll> _var,string _name=""){ cerr<<"Debug #set "<< _name<<" = ["; for(auto it : _var) cerr<<it<<","; cerr<<"END] : "<<"size = "<<_var.size()<<"\n\n";}
void light_debug(set<ll> _var,string _name=""){ cerr<<"Debug #set "<< _name<<" = ["; for(auto it : _var) cerr<<it<<","; cerr<<"END]\n\n";}
void debug(map<ll,ll> _var,string _name=""){ cerr<<"Debug #map (size = "<<_var.size()<<")\n"; for(auto it : _var) cerr<<_name<<"["<<it.first<<"] = "<<it.second<<'\n';}
void light_debug(map<ll,ll> _var,string _name=""){ cerr<<"Debug #map \n"; for(auto it : _var) cerr<<_name<<"["<<it.first<<"] = "<<it.second<<'\n';}
void debug(deque<ll> _var,string _name=""){ cerr<<"Debug #deque "<< _name<<" = ["; for(auto it : _var) cerr<<it<<","; cerr<<"END] : "<<"size = "<<_var.size()<<"\n\n";}
void light_debug(deque<ll> _var,string _name=""){ cerr<<"Debug #deque "<< _name<<" = ["; for(auto it : _var) cerr<<it<<","; cerr<<"END]\n\n";}
void debug(vector<ll> _var,string _name=""){ cerr<<"Debug #vector "<< _name<<" = ["; for(auto it : _var) cerr<<it<<","; cerr<<"END] : "<<"size = "<<_var.size()<<"\n\n";}
void light_debug(vector<ll> _var,string _name=""){ cerr<<"Debug #vector "<< _name<<" = ["; for(auto it : _var) cerr<<it<<","; cerr<<"END]\n\n";}
void debug(string _var,string _name=""){ cerr<<"Debug #string "<< _name<<" = "; cerr<<'"'<<_var<<'"'; cerr<<" : "<<"size = "<<_var.size()<<"\n\n";}
void light_debug(string _var,string _name=""){ cerr<<"Debug #string : "<<_name<<" = "<<'"'<<_var<<'"'<<"\n\n";}
void debug(ll _var,string _name=""){ cerr<<"Debug #int64 : "<<_name<<" = "<<_var<<"\n\n";}
void debug(ull _var,string _name=""){ cerr<<"Debug #uint64 : "<<_name<<" = "<<_var<<"\n\n";}
void debug(uint _var,string _name=""){ cerr<<"Debug #uint32 : "<<_name<<" = "<<_var<<"\n\n";}
void debug(int _var,string _name=""){ cerr<<"Debug #int32 : "<<_name<<" = "<<_var<<"\n\n";}
void debug(char _var,string _name=""){cerr<<"Debug #char : "<<_name<<" = '"<<_var<<"\n\n";}
void debug(bool _var,string _name=""){cerr<<"Debug #Boolean : "<<_name<<" = "<<(_var ? "1 True" : "0 False")<<"\n\n";}
void testt();
namespace andu2006{
    /// misc
    void multipleTests(){
        ll __test_count;
        cin>>__test_count;
        while(__test_count--)
            testt();
    }
    void fastIO(){
        ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
    }
    /// compare functions
    bool compare(ll a,ll b,char _cmp){
        if(_cmp == '<') return a < b;
        else return a > b;
    }
    bool compair(pll a, pll b, char _cmp1 = '<', char _cmp2 = '<'){ /// HAHA pun
        return compare(a.first, b.first, _cmp1) || (a.first == b.first && compare(a.second, b.second, _cmp2));
    }
    struct objcompair{
        bool operator () (pll a, pll b, char _cmp1 = '<', char _cmp2 = '<'){
            return compare(a.first, b.first, _cmp1) || (a.first == b.first && compare(a.second, b.second, _cmp2));
        }
    };
    /// binary functions
    ll trailingZeroes(ll x){
        if(x==0)
            return 0;
        return (x&1 ? 0 : 1 + trailingZeroes(x/2));
    }
    ll trailingOnes(ll x){
        if(x==0)
            return 0;
        return (x&1 ? 1 + trailingZeroes(x/2) : 0);
    }
    ll trailingPowerOfTwo(ll x){
        return 1ll<<trailingZeroes(x);
    }
    ll popCount(ll x){
        if(x == 0)
            return x;
        return 1 + popCount(x & (x - 1));
    }
    /// modular functions; "mod" MUST BE PRIME
    ll binPow(ll _num, ll _exponent, ll _mod){ /// a^b
        if(_exponent == 0)
            return 1;
        ll _half = binPow(_num, _exponent/2, _mod);

        return _half * _half % _mod * (_exponent & 1 ? _num : 1) % _mod;
    }
    ll modularInverse(ll _x, ll _mod){ /// (a / b ) % mod
        return binPow(_x, _mod - 2, _mod);
    }
    ll comb(ll _n, ll _k, ll _mod){  /// binom(a, b)
        ll _ans=1;

        for(ll i = _n - _k + 1; i <= _n; i++)
            _ans = _ans * i % _mod;

        for(ll i = 2; i <= _k; i++)
            _ans = _ans * modularInverse(i, _mod) % _mod;
        return _ans;
    }
    /// math
    const ld PI = acos(-1);
    const ld rad = 180 / PI;
    ld degsin(ll __alpha){
        return sin(__alpha / rad);
    }
    ld degcos(ll __alpha){
        return cos(__alpha / rad);
    }
    ld degtan(ll __alpha){
        return tan(__alpha / rad);
    }
    ld degasin(ll _sine){
        return asin(_sine) * rad;
    }
    ld degacos(ll _cosine){
        return acos(_cosine) * rad;
    }
    ld degatan(ll _tangent){
        return atan(_tangent) * rad;
    }
    ll divisors(ll _num){ /// number of divisors

        if(_num == 0) return 0;

        ll _cnt = 1, _exp;

        while(_num % 2==0){
            _cnt++;
            _num/=2;
        }
        for(ll _divisor = 3; _divisor * _divisor <= _num; _divisor += 2){
            _exp=0;
            while(_num % _divisor==0){
                _exp++;
                _num/=_divisor;
            }
            _cnt *= (_exp + 1);
        }
        if(_num>1) _cnt *= 2;
        return _cnt;
    }
    ll eulerTotient(ll _num){ /// computes the euler totient function
        if(_num == 0) return 0;

        ll _cnt = 1, _divisor=2;

        while(_num % _divisor == 0){
            _cnt *= _divisor;
            _num /= _divisor;
        }
        _cnt /= _divisor;
        _cnt *= (_divisor-1);
        for(ll _divisor = 3; _divisor * _divisor <= _num; _divisor += 2){
            if(_num % _divisor ==0){
                while(_num % _divisor == 0){
                    _cnt *= _divisor;
                    _num /= _divisor;
                }
                _cnt /= _divisor;
                _cnt *= (_divisor - 1);
            }
        }
        if(_num>1) _cnt *= _num-1;
        return _cnt;
    }
    void sieve(bool* _startpos, bool* _endpos){
         /// erathostenes' sieve; 0 means the number is prime; 1 means the number is composite
        ll _maxnum = _endpos - _startpos;
        for(ll i = 4; i < _maxnum; i += 2)
            *(_startpos+i)=1;
        for(ll i = 3; i * i < _maxnum; i+=2)
            if( *(_startpos + i) == 0)
                for(ll j = i * i; j< _maxnum; j+=i)
                    *(_startpos + j) = 1;
    }
}
/// - end of namespaces and start of program




#define NMAX 8001
#define MOD 1000000007
void testt()
{
}
int main()
{
    andu2006::fastIO();
    ll a,b,c,k,s=0;
    cin>>a>>b>>c>>k;
    s=min(a,k);
    k-=a;
    if(k<=0){
        cout<<s;
        return 0;
    }
    k-=b;
    if(k<=0){
        cout<<s;
        return 0;
    }
    cout<<s-k;
    return 0;
}


Measured memory_bytes: 3704