Language: C++

#include<iostream>
#include<stdio.h>
#include<vector>
#include<cmath>
using namespace std;
int a[20010],b[20010],begin[10000];
int main()
{
	int n,m,i,j,num,p,flag;
	while(cin>>n>>m){
		j=0;flag=0;
		for(i=1;i<=m;i++){
			cin>>a[i]>>b[i];
			if(a[i]==1)begin[++j]=i;
		}
		for(i=1;i<=j;i++){
			num=b[begin[j]];
			for(p=1;p<=m;p++){
				if(a[p]==num&&b[p]==n)flag=1;
			}
			if(flag){printf("POSSIBLE\n");break;}
		}
		if(flag==0)printf("IMPOSSIBLE\n");
	}
	return 0;
}


Compiler output:
a.cc: In function 'int main()':
a.cc:14:36: error: reference to 'begin' is ambiguous
   14 |                         if(a[i]==1)begin[++j]=i;
      |                                    ^~~~~
In file included from /usr/include/c++/14/string:53,
                 from /usr/include/c++/14/bits/locale_classes.h:40,
                 from /usr/include/c++/14/bits/ios_base.h:41,
                 from /usr/include/c++/14/ios:44,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:1:
/usr/include/c++/14/bits/range_access.h:114:37: note: candidates are: 'template<class _Tp> const _Tp* std::begin(const valarray<_Tp>&)'
  114 |   template<typename _Tp> const _Tp* begin(const valarray<_Tp>&) noexcept;
      |                                     ^~~~~
/usr/include/c++/14/bits/range_access.h:113:31: note:                 'template<class _Tp> _Tp* std::begin(valarray<_Tp>&)'
  113 |   template<typename _Tp> _Tp* begin(valarray<_Tp>&) noexcept;
      |                               ^~~~~
/usr/include/c++/14/bits/range_access.h:95:5: note:                 'template<class _Tp, long unsigned int _Nm> constexpr _Tp* std::begin(_Tp (&)[_Nm])'
   95 |     begin(_Tp (&__arr)[_Nm]) noexcept
      |     ^~~~~
/usr/include/c++/14/bits/range_access.h:63:5: note:                 'template<class _Container> constexpr decltype (__cont.begin()) std::begin(const _Container&)'
   63 |     begin(const _Container& __cont) -> decltype(__cont.begin())
      |     ^~~~~
/usr/include/c++/14/bits/range_access.h:52:5: note:                 'template<class _Container> constexpr decltype (__cont.begin()) std::begin(_Container&)'
   52 |     begin(_Container& __cont) -> decltype(__cont.begin())
      |     ^~~~~
In file included from /usr/include/c++/14/bits/range_access.h:36:
/usr/include/c++/14/initializer_list:88:5: note:                 'template<class _Tp> constexpr const _Tp* std::begin(initializer_list<_Tp>)'
   88 |     begin(initializer_list<_Tp> __ils) noexcept
      |     ^~~~~
a.cc:6:23: note:                 'int begin [10000]'
    6 | int a[20010],b[20010],begin[10000];
      |                       ^~~~~
a.cc:17:31: error: reference to 'begin' is ambiguous
   17 |                         num=b[begin[j]];
      |                               ^~~~~
/usr/include/c++/14/bits/range_access.h:114:37: note: candidates are: 'template<class _Tp> const _Tp* std::begin(const valarray<_Tp>&)'
  114 |   template<typename _Tp> const _Tp* begin(const valarray<_Tp>&) noexcept;
      |                                     ^~~~~
/usr/include/c++/14/bits/range_access.h:113:31: note:                 'template<class _Tp> _Tp* std::begin(valarray<_Tp>&)'
  113 |   template<typename _Tp> _Tp* begin(valarray<_Tp>&) noexcept;
      |                               ^~~~~
/usr/include/c++/14/bits/range_access.h:95:5: note:                 'template<class _Tp, long unsigned int _Nm> constexpr _Tp* std::begin(_Tp (&)[_Nm])'
   95 |     begin(_Tp (&__arr)[_Nm]) noexcept
      |     ^~~~~
/usr/include/c++/14/bits/range_access.h:63:5: note:                 'template<class _Container> constexpr decltype (__cont.begin()) std::begin(const _Container&)'
   63 |     begin(const _Container& __cont) -> decltype(__cont.begin())
      |     ^~~~~
/usr/include/c++/14/bits/range_access.h:52:5: note:                 'template<class _Container> constexpr decltype (__cont.begin()) std::begin(_Container&)'
   52 |     begin(_Container& __cont) -> decltype(__cont.begin())
      |     ^~~~~
/usr/include/c++/14/initializer_list:88:5: note:                 'template<class _Tp> constexpr const _Tp* std::begin(initializer_list<_Tp>)'
   88 |     begin(initializer_list<_Tp> __ils) noexcept
      |     ^~~~~
a.cc:6:23: note:                 'int begin [10000]'
    6 | int a[20010],b[20010],begin[10000];
      |                       ^~~~~