#include <fstream>
#include <iomanip>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
double ar;
struct point{
int x,y;
}p[100001];
int main()
{
    int n,i;
    fin>>n;
    for(i=1;i<=n;i++)
        fin>>p[i].x>>p[i].y;
    p[n+1]=p[1];
    ar=0;
    for(i=1;i<=n;i++)
        ar+=((p[i].y+p[i+1].y)*(p[i+1].x-p[i].x));
    if(ar<0) ar=-ar;
    ar=ar/2;
    //fout.precision(6);
    //fout<<fixed;
    fout<<ar;
    return 0;
}