#include <fstream>
#include <cmath>
#include <iostream>

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

typedef double ld;
struct point
{
    ld x;
    ld y;
};
ld trapez(point a,point b)
{
    return (a.y+b.y)*(b.x-a.x);
}
const int N=100000;
int n;
point v[N+5];

int main()
{
    fin>>n;
    for(int i=1;i<=n;i++)
        fin>>v[i].x>>v[i].y;
    v[n+1]=v[1];
    ld arie=0;
    for(int i=1;i<=n;i++)
        arie+=trapez(v[i],v[i+1]);
    fout<<fabs(0.5*arie);
    return 0;
}