using namespace std;
#include<fstream>
#include<iomanip>
#define NMax 100005
ifstream fin("aria.in");
ofstream fout("aria.out");
struct Point
{
	int x,y;
};
int N;
Point V[NMax];
double Sol;
void Read()
{
fin>>N;
for(int i=1;i<=N;i++)
	fin>>V[i].x>>V[i].y;
V[N+1]=V[1];
}
void Solve()
{
for(int i=1;i<=N;i++)
{
	Sol+=V[i].x*V[i+1].y-V[i].y*V[i+1].x;
}
if(Sol<0) Sol*=-1;
Sol/=2;
}
void Print()
{
fout<<setprecision(9)<<fixed<<Sol<<'\n';
}
int main()
{
	Read();
	Solve();
	Print();
	return 0;
}