#include <bits/stdc++.h>
#define PII pair<int,int>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
vector <PII>v;
long double sum=0;
int main()
{
    int n;
    PII a;
    fin>>n;
    for(int i=1;i<=n;i++)
    {
        fin>>a.first>>a.second;
        v.push_back(a);
    }
    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;
    fout<<sum;
    return 0;
}