#include <fstream>
#define M 100010
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
double x[M],y[M],aria;
int i,n;
int main()
{
    f>>n;
    for(i=1;i<=n;i++)
        f>>x[i]>>y[i];
    x[n+1]=x[1];y[n+1]=y[n];
    for(i=1;i<=n;i++)
        aria+= x[i]*y[i+1]-y[i]*x[i+1];
    if(aria <=-0.00000000001)aria = - aria;
    aria/=2.0;
    g<<aria;
    return 0;
}