#include <bits/stdc++.h>
#define point pair<double,double>
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
vector< point > a;
double aria;
int n;
int main()
{
    in>>n;
    a=vector< point >(n+2);
    for(int i=1;i<=n;i++)
        in>>a[i].first>>a[i].second;
    a[n+1]=a[1];
    for(int i=1;i<=n;i++)
        aria+=(a[i].first*a[i+1].second-a[i+1].first*a[i].second);
    out<<fixed<<setprecision(5)<<aria/2;
    return 0;
}