#include <bits/stdc++.h>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
typedef long double LD;
typedef pair<LD,LD> point;
vector <point>v;
point readPoint()
{
    LD x,y;
    f>>x>>y;
    return make_pair(x,y);
}
LD sum=0;
int n;
int main()
{
    f>>n;
    for(int i=1;i<=n;i++)
        v.push_back(readPoint());
    v.push_back(v[0]);
    for(int i=0;i<n;i++)
        sum+=v[i].first*v[i+1].second-v[i+1].first*v[i].second;
    sum/=2;
    if(sum<0)
        sum=-sum;
    g<<fixed<<setprecision(10)<<sum;
    return 0;
}