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

#define endl '\n'
#define rep(n)      for (int i = 0 ; i < n ; i++)
#define all(a)      a.begin(),a.end()
#define NumofDigits(n)    ((long long)log10(n)+1)
#define Unique(n)         (n).erase(unique(all(n)), (n).end())

typedef long long ll ;

/*            PANDA

                              _,add8ba,
                            ,d888888888b,
                           d8888888888888b                        _,ad8ba,_
                          d888888888888888)                     ,d888888888b,
                          I8888888888888888 _________          ,8888888888888b
                __________`Y88888888888888P"""""""""""baaa,__ ,888888888888888,
            ,adP"""""""""""9888888888P""^                 ^""Y8888888888888888I
         ,a8"^           ,d888P"888P^                           ^"Y8888888888P'
       ,a8^            ,d8888'                                     ^Y8888888P'
      a88'           ,d8888P'                                        I88P"^
    ,d88'           d88888P'                                          "b,
   ,d88'           d888888'                                            `b,
  ,d88'           d888888I                                              `b,
  d88I           ,8888888'            ___                                `b,
 ,888'           d8888888          ,d88888b,              ____            `b,
 d888           ,8888888I         d88888888b,           ,d8888b,           `b
,8888           I8888888I        d8888888888I          ,88888888b           8,
I8888           88888888b       d88888888888'          8888888888b          8I
d8886           888888888       Y888888888P'           Y8888888888,        ,8b
88888b          I88888888b      `Y8888888^             `Y888888888I        d88,
Y88888b         `888888888b,      `""""^                `Y8888888P'       d888I
`888888b         88888888888b,                           `Y8888P^        d88888
 Y888888b       ,8888888888888ba,_          _______        `""^        ,d888888
 I8888888b,    ,888888888888888888ba,_     d88888888b               ,ad8888888I
 `888888888b,  I8888888888888888888888b,    ^"Y888P"^      ____.,ad88888888888I
  88888888888b,`888888888888888888888888b,     ""      ad888888888888888888888'
  8888888888888698888888888888888888888888b_,ad88ba,_,d88888888888888888888888
  88888888888888888888888888888888888888888b,`"""^ d8888888888888888888888888I
  8888888888888888888888888888888888888888888baaad888888888888888888888888888'
  Y8888888888888888888888888888888888888888888888888888888888888888888888888P
  I888888888888888888888888888888888888888888888P^  ^Y8888888888888888888888'
  `Y88888888888888888P88888888888888888888888888'     ^88888888888888888888I
   `Y8888888888888888 `8888888888888888888888888       8888888888888888888P'
    `Y888888888888888  `888888888888888888888888,     ,888888888888888888P'
     `Y88888888888888b  `88888888888888888888888I     I888888888888888888'
       "Y8888888888888b  `8888888888888888888888I     I88888888888888888'
         "Y88888888888P   `888888888888888888888b     d8888888888888888'
            ^""""""""^     `Y88888888888888888888,    888888888888888P'
                             "8888888888888888888b,   Y888888888888P^
                              `Y888888888888888888b   `Y8888888P"^
                                "Y8888888888888888P     `""""^
                                  `"YY88888888888P'
                                       ^""""""""'
  */
void quick ()
{
    cin.tie(0);
    cout.tie(0);
    ios_base::sync_with_stdio(0);
}

void readFromText ()
{
#ifndef ONLINE_JUDGE
    freopen("input.txt","r",stdin);
#endif // ONLINE_JUDGE
}

const double Pi = acos(-1.0);


const int MAX = 1e6 ;
int arr[MAX];

int main()
{
    quick();
//    readFromText();
    long long a , b , c , k ; cin >> a >> b >> c >> k ;
    long long sum = 0 ;
    if (k<=a){
        return cout << k , 0 ;
    }else {
        sum+=a;
        k-=a;
    }
    if (k<=b){
        return cout << sum , 0 ;
    }else {
        k-=b;
    }
    sum-=k;
    cout << sum << endl ;
    return 0;
}



Measured memory_bytes: 3624