#include <bits/stdc++.h>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
long double xs,ys,xc,yc,xv,yv,a;
int n;
int main()
{
    f>>n>>xs>>ys;
    for(xc=xs,yc=ys,n--;n;n--,xc=xv,yc=yv)
    {
        f>>xv>>yv;
        a+=(xc*yv-xv*yc)/2;
    }
    a+=(xc*ys-xs*yc)/2;
    if(a<0)a=-a;
    g<<fixed<<setprecision(10)<<a;
    return 0;
}