#include<fstream>
using namespace std;
struct puncte{double x,y;};
ifstream fin("aria.in");
ofstream fout("aria.out");
int main()
{long i,n;
puncte v[10000];
double aria=0;
fin>>n;
for(i=1;i<=n;i++)
   fin>>v[i].x>>v[i].y;
   for(i=1;i<=n;i++)
aria=aria+v[i].x*v[i+1].y-v[i+1].x*v[i].y;
aria=aria+v[n].x*v[1].y-v[1].x*v[n].y;
if(aria>0)
    fout<<aria/2.0;
else
fout<<aria*(-0.5);
    return 0;
}