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Cod sursa(job #670436)
#include <fstream>
#include <algorithm>
using namespace std;
ifstream fin("infasuratoare.in");
ofstream fout("infasuratoare.out");
#define dim 120005
#define inf 0x3f3f3f3f
#define fs first
#define sc second
pair <double,double>v[dim];
int n, orig=1,ind[dim], st[dim],vf;
inline double panta(int a, int b)
{
if(v[a].fs==v[b].fs)
return inf;
return (v[a].sc-v[b].sc)/(v[a].fs-v[b].fs);
}
struct cmp
{
bool operator()(const int &a, const int &b)const
{
return panta(a,orig)<panta(b,orig);
}
};
void read()
{
fin>>n;
for(int i=1;i<=n;++i)
{
fin>>v[i].fs >>v[i].sc;
if(v[i].fs<v[orig].fs || (v[i].fs==v[orig].fs && v[i].sc<v[orig].sc))
orig=i;
ind[i]=i;
}
}
inline double det(int a, int b, int c)
{
return (v[b].fs-v[a].fs)*(v[c].sc-v[a].sc)-(v[c].fs-v[a].fs)*(v[b].sc-v[a].sc);
}
void solve()
{
sort(ind+1,ind+n+1,cmp());
st[++vf]=orig;
for(int i=1;i<=n;++i)
if(ind[i]!=orig)
{
for(;vf>=2 && det(st[vf-1],st[vf],ind[i])<0;--vf);
st[++vf]=ind[i];
}
}
void printf()
{
fout<<vf<<'\n';
for(int i=1;i<=vf;++i)
fout<<v[st[i]].fs <<" " <<v[st[i]].sc <<'\n';
}
int main()
{
read();
solve();
printf();
return 0;
}