#include <iostream>
#include <fstream>
#include <cmath>
#include <iomanip>
using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");
struct p{
double x,y;
}
v[100010];
long double ras;
    int i,n;
int main()
{


    fin>>n;
    for(i=1;i<=n;i++)
        fin>>v[i].x>>v[i].y;
    v[n+1].x=v[1].x;
    v[n+1].y=v[1].y;
    for(i=1;i<=n;i++)
        ras=ras+(v[i].x*v[i+1].y-v[i].y*v[i+1].x);
    if(ras<0)ras=ras/-2.0;
    else ras=ras/2.0;
    fout<<setprecision(10)<<ras<<'\n';
    return 0;
}