#include <bits/stdc++.h>
using namespace std;
ifstream fin ("aria.in");
ofstream fout("aria.out");
struct point
{
    double x,y;
}v[100010];
double area;
int n,i,j;
double makeArie(point a,point b,point c)
{
    return abs(a.x*b.y+b.x*c.y+c.x*a.y-b.y*c.x-c.y*a.x-a.y*b.x);
}
int main()
{
    fin>>n;
    for(i=1;i<=n;i++)
        fin>>v[i].x>>v[i].y;
    for(i=1;i<=n-2;i++)
        area=area+makeArie(v[i],v[i+1],v[i+2]);
    fout<<area/2;
    return 0;
}