#include <fstream>
#include <iostream>
#include <math.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;
    cin>>n;
    cin>>pct[1].x>>pct[1].y;
    //a = a + pct[1].x * pct[2].y;
    //b = b + pct[2].x * pct[1].y;
    pct[n+1].x = pct[1].x;
    pct[n+1].y = pct[1].y;
    for(int i = 2; i <= n; i++)
    {
        cin>>pct[i].x>>pct[i].y;
        a = a + pct[i-1].x * pct[i].y;
        b = b + pct[i].x * pct[i-1].y;
    }
    a = a + pct[n].x * pct[n+1].y;
    b = b + pct[n+1].x * pct[n].y;
    cout << (a - b) / 2;
    return 0;
}