#include <iostream>
#include <math.h>
#include <fstream>
#include <iomanip>
#include <stdio.h>
#include <stdlib.h>
using namespace std;
ifstream fin("aria.in");
ofstream g("aria.out");
struct puncte{double x ; double y;}v[100001];;
long long int i,j,n;
float a,b,c,p,s;
int main()
{
    fin>>n;
    for(i=1;i<=n;i++)
        fin>>v[i].x>>v[i].y;
        v[n+1]=v[1];
    for(i=1;i<=n;i++)
    {
        s+=v[i].x*v[i+1].y-v[i+1].x*v[i].y;
        if(s<0) s=-1*s;
    }
    printf("%.6f",s/2);
}