#include<fstream>
#define punct pair<double,double>
#define x first
#define y second
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int i,n;
punct v[100100];
inline double abs(double x)
{
    if(x<0)
    return -x;
    return x;
}
double aria;
int main()
{
    f>>n;
    for(i=1;i<=n;++i)
    f>>v[i].x>>v[i].y;
    v[n+1]=v[1];
    v[0]=v[n];
    for(i=1;i<=n;++i)
    aria=aria+1.0*v[i].x*(v[i+1].y-v[i-1].y);
    aria/=2;
    aria=abs(aria);
    g<<aria<<'\n';
    return 0;
}