Language: C++

#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef long double ld;
typedef vector<ll> vl;
typedef vector<string> vs;
typedef vector<char> vc;
typedef queue<ll> ql;
typedef deque<ll> dql;
typedef priority_queue<ll/*, vl, greater<ll>*/> pql; //降順(/*昇順*/)
typedef set<ll> sl;
typedef pair<ll, ll> pl;
typedef vector<vl> vvl;
typedef vector<pl> vpl;
#define rep(i, n) for(ll i = 0; i < ll(n); i++)
#define rep2(i, n) for(ll i = 1; i <= ll(n); i++)
//#define rep(i, k, n) for(ll i = k-1; i < ll(n); i++)
//#define rep2(i, k, n) for(ll i = k; i <= ll(n); i++)
#define all(v) (v).begin(), (v).end()
bool chmin(ll &a, ll b) {if(b < a) {a = b; return 1;} return 0;}
bool chmax(ll &a, ll b) {if(b > a) {a = b; return 1;} return 0;}
const ll INF = 1LL << 60;
const ll MOD = 1e9 + 7;
//const ll MOD = 998244353;
const ll MAX = 1e9;
const char newl = '\n';

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);

  ll n, m;
  cin >> n >> m;
  
  vl G(n);
  ll a, b;
  rep(_, m) {
    cin >> a >> b;
    a--; b--;
    G[a].push_back(b);
    G[b].push_back(a);
  }
  
  for(ll i: G[0]) for(ll j: G[i]) {
    if(j==n-1) {cout << "POSSIBLE" << newl; return 0;}
  }
  
  cout << "IMPOSSIBLE" << newl;
  return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:40:10: error: request for member 'push_back' in 'G.std::vector<long long int>::operator[](((std::vector<long long int>::size_type)a))', which is of non-class type '__gnu_cxx::__alloc_traits<std::allocator<long long int>, long long int>::value_type' {aka 'long long int'}
   40 |     G[a].push_back(b);
      |          ^~~~~~~~~
a.cc:41:10: error: request for member 'push_back' in 'G.std::vector<long long int>::operator[](((std::vector<long long int>::size_type)b))', which is of non-class type '__gnu_cxx::__alloc_traits<std::allocator<long long int>, long long int>::value_type' {aka 'long long int'}
   41 |     G[b].push_back(a);
      |          ^~~~~~~~~
a.cc:44:16: error: 'begin' was not declared in this scope
   44 |   for(ll i: G[0]) for(ll j: G[i]) {
      |                ^
a.cc:44:16: note: suggested alternatives:
In file included from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:166,
                 from a.cc:1:
/usr/include/c++/14/valarray:1238:5: note:   'std::begin'
 1238 |     begin(const valarray<_Tp>& __va) noexcept
      |     ^~~~~
In file included from /usr/include/c++/14/filesystem:53,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:200:
/usr/include/c++/14/bits/fs_dir.h:608:3: note:   'std::filesystem::__cxx11::begin'
  608 |   begin(recursive_directory_iterator __iter) noexcept
      |   ^~~~~
a.cc:44:16: error: 'end' was not declared in this scope
   44 |   for(ll i: G[0]) for(ll j: G[i]) {
      |                ^
a.cc:44:16: note: suggested alternatives:
/usr/include/c++/14/valarray:1265:5: note:   'std::end'
 1265 |     end(const valarray<_Tp>& __va) noexcept
      |     ^~~
/usr/include/c++/14/bits/fs_dir.h:613:3: note:   'std::filesystem::__cxx11::end'
  613 |   end(recursive_directory_iterator) noexcept
      |   ^~~
a.cc:44:32: error: 'begin' was not declared in this scope
   44 |   for(ll i: G[0]) for(ll j: G[i]) {
      |                                ^
a.cc:44:32: note: suggested alternatives:
/usr/include/c++/14/valarray:1238:5: note:   'std::begin'
 1238 |     begin(const valarray<_Tp>& __va) noexcept
      |     ^~~~~
/usr/include/c++/14/bits/fs_dir.h:608:3: note:   'std::filesystem::__cxx11::begin'
  608 |   begin(recursive_directory_iterator __iter) noexcept
      |   ^~~~~
a.cc:44:32: error: 'end' was not declared in this scope
   44 |   for(ll i: G[0]) for(ll j: G[i]) {
      |                                ^
a.cc:44:32: note: suggested alternatives:
/usr/include/c++/14/valarray:1265:5: note:   'std::end'
 1265 |     end(const valarray<_Tp>& __va) noexcept
      |     ^~~
/usr/include/c++/14/bits/fs_dir.h:613:3: note:   'std::filesystem::__cxx11::end'
  613 |   end(recursive_directory_iterator) noexcept
      |   ^~~