#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
struct poz {double x;double y;}v[100010];
int i,n;
double a;
int main()
{
    in>>n;
    for(i=1;i<=n;i++)
        in>>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++)
        a+=v[i].x*v[i+1].y-v[i].y*v[i+1].x;
    a=a/2;
    if(a<0)
        a=-a;
    out<<fixed<<setprecision(10)<<a<<'\n';
    in.close();
    out.close();
    return 0;
}