#include <iostream>
#include <iomanip>
#include <fstream>
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
int n;
long double a,b;
pair<long double,long double> v,v2;
long double dp[100005];
long double rez;
int main()
{
    in>>n;
    in>>a>>b;
    v2.first=a;
    v2.second=b;
    for(int i=2;i<=n;i++)
    {
        in>>v.first>>v.second;
        dp[i-1]=(v2.first*v.second-v.first*v2.second)/2;
        v2.first=v.first;
        v2.second=v.second;
    }
    dp[n]=(v2.first*b-a*v2.second)/2;
    for(int i=1;i<=n;i++)
    {
        rez+=dp[i];
    }
    out<<setprecision(6)<<rez;
    return 0;
}