Language: C++

#define debug_interval ','
#define dump_interval ' '
#define debug_toggle 1
//{
#include<bits/stdc++.h>
using namespace std;
#define hi cout<<"hi"<<endl;
#define int long long
#define INT_MAX LLONG_MAX
#define rep(i,n) for(int i=0;i<(n);i++)
#define rep1(i,n) for(int i=1;i<=(n);i++)
#define pb push_back
#define all(v) (v).begin(),(v).end()
#define rall(v) (v).rbegin(),(v).rend()
#define fi first
#define se second
#define mp make_pair
#define rev reverse
#define dans dump(ans)
#define MOD 1000000007
#define amp(v,n) (v).count(n)?v[n]++:v[n]=1;

//{
inline int toInt(string s){int v;istringstream sin(s);sin>>v;return v;}
template<class T> inline string toString(T x){ostringstream sout;sout<<x;return sout.str();}
template<class...A> inline void dump(){cout<<endl;}
template<class...A> inline void dump_rest() {cout<<endl;}
template<class T, class...A> inline void dump_rest(const T& first, const A&... rest){cout<<dump_interval<<first;dump_rest(rest...);}
template<class T,class...A> inline void dump(const T&first,const A&...rest){cout<<first;dump_rest(rest...);}
template<class...A> inline void debug(){cerr<<endl;}
template<class...A> inline void debug_rest() {cerr<<endl;}
template<class T, class...A> inline void debug_rest(const T& first, const A&... rest){cerr<<debug_interval<<first;debug_rest(rest...);}
template<class T,class...A> inline void debug(const T&first,const A&...rest){if(debug_toggle)cerr<<first,debug_rest(rest...);}
//}

typedef vector<int>vint;
typedef pair<int,int>pint;
typedef vector<pint>vpint;

template<typename A,typename B>inline void chmin(A &a,B b){if(a>b)a=b;}
template<typename A,typename B>inline void chmax(A &a,B b){if(a<b)a=b;}
template<int m>class mint{private:int i;public:mint() : i(0){}mint(int i): i((i%m+m)%m){}mint operator+(const mint& o){return o.i+i;}mint operator*(const mint& o){return o.i*i;}mint operator-(){return -i;}operator int() {return i;}};

int n,m,a,b;
set<int> l,r;

main(){
    cin>>n>>m;
    rep(i,m){
        cin>>a>>b;
        if(a==1)l.insert(b);
        if(b==n)r.insert(a);
    }
    for(auto x:b){
        if(r.count(x)){
            dump("POSSIBLE");
            return 0;
        }
    }
    dump("IMPOSSIBLE");
}



Compiler output:
a.cc:9:9: warning: "INT_MAX" redefined
    9 | #define INT_MAX LLONG_MAX
      |         ^~~~~~~
In file included from /usr/include/c++/14/climits:42,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:38,
                 from a.cc:5:
/usr/lib/gcc/x86_64-linux-gnu/14/include/limits.h:120:9: note: this is the location of the previous definition
  120 | #define INT_MAX __INT_MAX__
      |         ^~~~~~~
a.cc:47:1: warning: ISO C++ forbids declaration of 'main' with no type [-Wreturn-type]
   47 | main(){
      | ^~~~
a.cc: In function 'int main()':
a.cc:54:16: error: 'begin' was not declared in this scope
   54 |     for(auto x:b){
      |                ^
a.cc:54:16: note: suggested alternatives:
In file included from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:166:
/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:54:16: error: 'end' was not declared in this scope
   54 |     for(auto x:b){
      |                ^
a.cc:54:16: 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
      |   ^~~