#include<fstream>
using namespace std;
struct punct
{
    double x,y;
}a[100001];
punct p1,p2,po;
int i,n;
double s;
int main()
{
ifstream fin("aria.in");
ofstream fout("aria.out");
    fin>>n;
    for(i=1;i<=n;i++)
    fin>>a[i].x>>a[i].y;
    a[n+1]=a[1];
    for(i=1;i<=n;i++)
    s+=(a[i].y+a[i+1].y)*(a[i+1].x-a[i].x);
    if(s<0) s=-s;
    fout<<fixed<<s/2;
}