#include<fstream>
#include<iomanip>
using namespace std;
int n;
double a,b,c,d,x,y;
long double s=0;
int main()
{
    ifstream f("aria.in");
    ofstream g("aria.out");
    f>>n>>x>>y,a=c=x,b=d=y,n--;
    while(n--)
        f>>x>>y,s+=(long double)a*y-(long double)x*b,a=x,b=y;
    s+=(long double)a*d-(long double)b*c;
    s/=2;
    g<<setprecision(6)<<fixed<<s;
}