Language: C++

#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
#define FOR(i,l,r) for(i=l;i<r;i++)
#define REP(i,n) FOR(i,0,n)
#define ALL(x) x.begin(),x.end()
#define P pair<ll,ll>
#define F first
#define S second
signed main(){
  string S;int K,i,j,k=0;cin>>S>>K;
  vector<string>SUB(S.size()*(S.size()+1)/2);
  REP(i,S.size())FOR(j,1,min(K,S.size()-i+1))SUB.at(k++)=S.substr(i,j);
  sort(ALL(SUB),[](const string&x,const string&y){return x<y;});j=0;
  REP(i,K-1){
    j++;
    while(SUB.at(j-1)==SUB.at(j))j++;
  }
  cout<<SUB.at(j)<<endl;
  return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:13:29: error: no matching function for call to 'min(int&, std::__cxx11::basic_string<char>::size_type)'
   13 |   REP(i,S.size())FOR(j,1,min(K,S.size()-i+1))SUB.at(k++)=S.substr(i,j);
      |                          ~~~^~~~~~~~~~~~~~~~
a.cc:4:30: note: in definition of macro 'FOR'
    4 | #define FOR(i,l,r) for(i=l;i<r;i++)
      |                              ^
In file included from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_algobase.h:233:5: note: candidate: 'template<class _Tp> constexpr const _Tp& std::min(const _Tp&, const _Tp&)'
  233 |     min(const _Tp& __a, const _Tp& __b)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:233:5: note:   template argument deduction/substitution failed:
a.cc:13:29: note:   deduced conflicting types for parameter 'const _Tp' ('int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'})
   13 |   REP(i,S.size())FOR(j,1,min(K,S.size()-i+1))SUB.at(k++)=S.substr(i,j);
      |                          ~~~^~~~~~~~~~~~~~~~
a.cc:4:30: note: in definition of macro 'FOR'
    4 | #define FOR(i,l,r) for(i=l;i<r;i++)
      |                              ^
/usr/include/c++/14/bits/stl_algobase.h:281:5: note: candidate: 'template<class _Tp, class _Compare> constexpr const _Tp& std::min(const _Tp&, const _Tp&, _Compare)'
  281 |     min(const _Tp& __a, const _Tp& __b, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:281:5: note:   candidate expects 3 arguments, 2 provided
In file included from /usr/include/c++/14/algorithm:61:
/usr/include/c++/14/bits/stl_algo.h:5686:5: note: candidate: 'template<class _Tp> constexpr _Tp std::min(initializer_list<_Tp>)'
 5686 |     min(initializer_list<_Tp> __l)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5686:5: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_algo.h:5696:5: note: candidate: 'template<class _Tp, class _Compare> constexpr _Tp std::min(initializer_list<_Tp>, _Compare)'
 5696 |     min(initializer_list<_Tp> __l, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5696:5: note:   template argument deduction/substitution failed:
a.cc:13:29: note:   mismatched types 'std::initializer_list<_Tp>' and 'int'
   13 |   REP(i,S.size())FOR(j,1,min(K,S.size()-i+1))SUB.at(k++)=S.substr(i,j);
      |                          ~~~^~~~~~~~~~~~~~~~
a.cc:4:30: note: in definition of macro 'FOR'
    4 | #define FOR(i,l,r) for(i=l;i<r;i++)
      |                              ^