#include <stdio.h>
#include <cmath>
using namespace std;
FILE *f,*g;
struct PCT
{
    double x,y;
};
PCT v[100010],G,A,B;
int N;
void Read()
{
    int i;
    fscanf(f,"%d",&N);
    for(i=1;i<=N;++i)
    {
        fscanf(f,"%lf %lf",&v[i].x,&v[i].y);
        G.x+=v[i].x,G.y+=v[i].y;
    }
    G.x/=2;G.y/=2;
}
double Det(int Ps,int Pd)
{
    A=v[Ps];B=v[Pd];
    return abs((A.x*B.y+B.x*G.y+G.x*A.y-B.y*G.x-G.y*A.x-A.y*B.x));
}
void Solve()
{
    int i;
    v[0]=v[N];
    double Area=0;
    for(i=1;i<=N;++i)
        Area+=Det(i-1,i);
    Area/=2.0;
    fprintf(g,"%0.6lf",Area);
}
int main()
{
    f=fopen("aria.in","r");g=fopen("aria.out","w");
    Read();Solve();
    fclose(f);fclose(g);
    return 0;
}