/*
--------------------------------DO NOT COPY I REQUEST YOU PLEASE--------------------------

AUTHOR : Chandan Agrawal
College : Poornima College of Engg. jaipur, Raj
Mail : chandanagrawal23@gmail.com


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*/


#include<bits/stdc++.h>
#include<stdio.h>
using namespace std;

#define fastio ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define MAX 100050

#define ll long long
#define ld long double
#define lli long long int

#define pb emplace_back
#define INF 1000000000
#define mod 10000000019
#define MOD 10000000019
//#define mp make_pair
#define loop(i,n) for (lli i = 0; i < n; i++)
#define loopitr(xt,vec) for (auto xt : vec)
#define FOR(i,a,b) for (lli i = a; i < b; i+=1)
#define loop_rev(i,n) for (lli i = n-1; i >= 0; i--)
#define FOR_REV(i,a,b) for (lli i = a; i >= b; i--)
#define all(v) v.begin(),v.end()
#define sz(x) int(x.size())
#define pii pair<int,int>
#define F first
#define S second

#define mii map<lli,lli>
#define vi vector<lli>
#define seti set<lli>

#define itr :: iterator it
#define WL(t) while(t --)
#define gcd(a,b) __gcd((a),(b))
#define lcm(a,b) (a/gcd(a,b))*b
#define abs(x) ((x < 0)?-(x):x)

#define print(x) printf("%lli\n",x);
#define print2(x,y) printf("%lli %lli\n",x,y);
#define print3(x,y,z) printf("%lli %lli %lli\n",x,y,z);

#define scan(x) scanf("%lld",&x);
#define scan2(x,y) scanf("%lld %lld",&x,&y);
#define scan3(x,y,z) scanf("%lld %lld %lld",&x,&y,&z);

#define scanarr(a,n) for(lli i=0;i<n;i++)    cin>>a[i];
#define scanvector(a,n) for(lli i=0;i<n;i++){ lli x ; cin>>x; a.push_back(x);}

#define printarr(a,n) for(lli i=0;i<n;i++)   printf("%lli ",a[i]); printf("\n");
#define printvector(vec) for(auto xt : vec) cout<<xt<<" ";    cout<<"\n";
#define printset(st) for(auto xt : st) cout<<xt<<" ";    cout<<"\n";

#define FD(N) fixed<<setprecision(N)

#define endl '\n'

#define deb(x) cout<<#x<<" "<<x<<endl;

/*
ifstream cinn("i3.txt");
ofstream coutt("o3.txt");
*/


lli find_mod(string s){
lli i,x=0;
for(i=0;i<s.length();i++)
    x=(x*10+s[i]-'0')%MOD;
return x;
}


bool prime[MAX+1];

void sieve1(){ //prime or not
    for(int i=0;i<=MAX;i++) prime[i]=1;
    prime[0]=prime[1]=0;
    for(lli i=4;i<=MAX;i+=2) prime[i]=0;
    for (int p=3; p*p<=MAX; p+=2){
        if (prime[p] == 1)
            for (int i=p*2; i<=MAX; i += p){
                prime[i] = 0;
}}}

lli spf[MAX+1];

void sieve2(){ //smallest prime factor of N.

    loop(i,MAX+1) spf[i]=i;
    for(lli i=4;i<=MAX;i+=2) spf[i]=2;
    
    for(lli i=3; i*i<=MAX; i++){ 
        if (spf[i] == i){ 
            for (int j=i*i; j<=MAX; j+=i)  
                if (spf[j]==j) //marking spf[j] if it is not previously marked 
                    spf[j] = i; 
        } 
    } 
}

lli lpf[MAX+1];

void sieve3(){ //largest prime factor of N
    loop(i,MAX+1) lpf[i]=i;
    for(lli i=4;i<=MAX;i+=2) lpf[i]=2;
    
    for(lli i=3; i*i<=MAX; i++) { 
        if (lpf[i] == i){ 
            for (int j=2*i; j<=MAX; j+=i)  
                    lpf[j] = i; 
        } 
    } 
    
    
}


bool is_even(lli n){ return(!(n&2));}

bool is_odd(lli n){ return(n&2);}

// memset(array_name , initialized value , sizeof(data_type_of_array))

bool isPrime(lli n){
if(n<=1) return false;
if(n<=3) return true;
if(n==5) return true;
if((n%2==0)||(n%3==0) || (n%5==0)) return false;
for(lli i=5;i*i<=n;i+=6)
    if((n%i==0)||(n%(i+2)==0))
        return false;
return true;
}

lli reversenum(lli x){
lli num=0;
while(x>0){
    num=num*10+x%10;
    x=x/10;
    }
return num;
}

bool ispalindrome(string s){
string q = s;
reverse(q.begin(),q.end());
return(s==q);
}

bool isvowel(char x){    return(x=='a' || x== 'e' ||x=='i' || x== 'o' || x=='u' );    }

lli mceil(lli a,lli b){
    if(a%b==0) return(a/b);
    else return(a/b  +1);
}
lli mfloor(lli a,lli b){
    return(a/b);
}
lli sumarr(lli a[],lli n){
    lli sum=0;
    loop(i,n) sum+=a[i];
    return sum;
}
ll modmullong(ll a,ll b){
    ll  res = 0;
    a %= mod;
    while (b){
        if (b & 1)
            res = (res + a) % mod;
        a = (2 * a) % mod;
        b >>= 1;
    }
    return res;
}
ll modmul(ll a, ll b) {
    return ((a%mod) * (b%mod)) % mod;
}
ll modmul2(ll a, ll b) {
    return (((a%(mod-1)) * (b%(mod-1))) % (mod-1));
}

ll modadd(ll a , ll b){
    return((a%mod)+(b%mod)+mod)%mod;
}

ll modsub(ll a , ll b){
    return((a%mod) - (b%mod) + mod)%mod;
}

lli fastexpo(lli a,lli b){
    a = a%mod;
    lli ans=1;
    while(b){
        if(b&1)
            ans=(ans*1ll*a)%mod;
        a=(a*1ll*a)%mod;
        b=b/2;
        }
    return ans;
 }

lli phi[MAX];
void euler_phi(){ //euler totient function
loop(i,MAX-1)
    phi[i+1] = i+1;
for (lli p=2; p<MAX; p++){
    if (phi[p] == p){
        phi[p] = p-1;

        for (lli i = 2*p; i<MAX; i += p)
               phi[i] = (phi[i]/p) * (p-1);

        }
    }
}

lli myphi(lli n)
{
    lli result = n;
    for (lli p = 2; p * p <= n; ++p) {
        if (n % p == 0) {
            while (n % p == 0)
                n /= p;
            result -= result / p;
        }
    }
    if (n > 1)
        result -= result / n;
    return result;
}


void prime_factorise(lli n , map<lli,lli> &mp){

    while(n%2==0){
        mp[2]++;
        n/=2;
    }
    for(lli i=3;i*i<=n;i+=2){
        while(n%i==0){
            mp[i]++;
            n/=i;
        }
    }

    if(n>1) mp[n]++;

 }

lli cntbits(lli n){
    lli cnt=0;
    while(n){
        cnt++;
        n/=2;
    }
    return cnt;
}

lli fact[100001];
void facto(){
    fact[0] = 1;
    for(lli i=1;i<=100000;i++)
        fact[i]  = modmul(fact[i-1],i);
}

lli nCr(lli n,lli r){
    if(n<r)
        return 0;
    else{
        r = min(r,n-r);
        lli a=1,b=1;
            for(lli i=0;i<r;i++) {
                a = (a*(n-i))%mod;
                b = (b*(i+1))%mod;
            }
            a = modmul(a,fastexpo(b,mod-2));
            return a;
    }
}
bool ispower2(lli n){
    return(n and (n&(n-1))==0 );
}

vector<string> split(string s,char ch){
    vector<string> ans;
    string temp="";
    for(lli i=0;i<sz(s);i++){
        if(s[i]==ch){
            ans.pb(temp);
            temp="";
        }
        else{
            temp+=s[i];
        }
    }
    ans.pb(temp);
    return ans;
}

// chandan1,2
void chandan1(){int y=1;return;}
void chandan2(){
        loop(i,10){
        lli x=1;
    }
    return(chandan1());
}

int main(){
fastio
lli t=1;
//cin>>t;
chandan2();
while(t--) {
    lli a,b,c,k;
    cin>>a>>b>>c>>k;
    lli sum=0;
    if(k<=a){
        sum=k;
    }
    else{
        sum = a;
        k-=a;
        if(k<=b){
            int xx=1;
        }
        else if(k>b){
            k-=b;
            sum-=(min(k,c));
        }
    }
    cout<<sum<<endl;
    
}

return 0;
}



Measured memory_bytes: 3668