#include <iostream>
#include <fstream>
#include <algorithm>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
double aria;
int n,i;
struct punct
{
    double x,y;
}a[100005];

int main()
{
    f>>n;
    for(i=1;i<=n;i++)
        f>>a[i].x>>a[i].y;
    a[n+1]=a[1];
    aria=0;
    for(i=1;i<=n;i++)
        aria+=(a[i].x*a[i+1].y-a[i+1].x*a[i].y);
    aria/=2.0;
    if(aria<0)
       aria=-aria;
    g<<aria;
    return 0;
}