#include<fstream>
#include<algorithm>
#include<iomanip>
using namespace std;
ifstream f ("aria.in");
ofstream g ("aria.out");
int n;
long double xa,ya,xb,yb,x,y,sum;

int main()
{
    f>>n;
    f>>xa>>ya;
    x=xa;
    y=ya;
    for(int i=2;i<=n;++i){
        f>>xb>>yb;
        sum+=(xa*yb-ya*xb);
        xa=xb;
        ya=yb;
    }
    sum+=(xa*y-ya*x);
    if(sum<0) sum=-sum;
    sum/=2;
    g<<fixed<<setprecision(6)<<sum;
    return 0;
}