#include <bits/stdc++.h>

using namespace std;

ifstream ci ("aria.in");
ofstream co ("aria.out");

int a[100001], b[100001];

int main()
{
    int n;
    double sum=0;
    ci >> n;
    for (int i=1; i<=n; i++)
    {
        ci >> a[i] >> b[i];
    }
    for (int i=1; i<n; i++)
    {
        sum+=(a[i+1]-a[i])*(b[i]+b[i+1]);
    }
    sum+=(a[1]-a[n])*(b[n]+b[1]);
    if (sum<0)
    {
        co << -sum/2;
        //co << setprecision(6) << fixed << -sum/2.0;
    }
    else
    {
        co << sum/2;
        //co << setprecision(6) << fixed << sum/2.0;
    }
    return 0;
}