#include <iostream>
#include <fstream>
#include <iomanip>
#define nmax 100999
using namespace std;

struct punct{double x,y;}p[nmax];

long double abs(long double x){if(x>=0)return x;return -x;}
ifstream fin("aria.in");
ofstream fout("aria.out");
int main()
{int i,n;
long double arie=0;
    fin>>n;
    for(i=1;i<=n;i++)
        fin>>p[i].x>>p[i].y;
    p[0]=p[n];p[n+1]=p[1];

    for(i=1;i<=n;i++)
        arie+=p[i].x*(p[i+1].y-p[i-1].y);
    fout<<fixed<<setprecision(7)<<abs(arie/2);
}