#include <iostream>
#include <iomanip>
#include <fstream>
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
int n;
pair<long long,long long> v[100005];
long double dp[100005];
long double rez;
int main()
{
    in>>n;
    for(int i=1;i<=n;i++)
    {
        in>>v[i].first>>v[i].second;
    }
    for(int i=1;i<n;i++)
    {
        dp[i]=(v[i].first*v[i+1].second-v[i+1].first*v[i].second)/2;
    }
    dp[n]=(v[n].first*v[1].second-v[1].first*v[n].second)/2;
    for(int i=1;i<=n;i++)
    {
        rez+=dp[i];
    }
    out<<setprecision(6)<<rez;
    return 0;
}