#include <bits/stdc++.h>

using namespace std;

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

struct punct
{
    long double x,y;
}pct[100002];

int main()
{
    long double a = 0, b = 0;
    int n;
    in>>n;
    for(int i = 1; i <= n; i++)
        in>>pct[i].x>>pct[i].y;
    pct[n+1].x = pct[1].x;
    pct[n+1].y = pct[1].y;
    for(int i = 1; i <= n; i++)
    {
        a = a + pct[i].x * pct[i+1].y - pct[i+1].x * pct[i].y;;
    }
    out << fixed<<setprecision(6)<<a/(long double)2;
    return 0;
}