#include<fstream>
#include<iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int n,i,j,x[100005],y[100005];
float s;
int main ()
{
    f>>n;
    for(i=1;i<=n;i++) f>>x[i]>>y[i];
    x[n+1]=x[1];
    y[n+1]=y[1];

    for(i=1;i<=n+1;i++)
    {
        s+=x[i+1]*y[i]-x[i]*y[i+1];
    }
    if(s<0) s*=-1;

    g<<s/2;

    return 0;
}