#include <bits/stdc++.h>

using namespace std;

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

struct punct
{
    long double x,y;
};
punct a[100002];
long double arie;
int n;

long double Aria(punct a,punct b)
{
    return (a.x*b.y-b.x*a.y)*0.5;
}

int main()
{
    in>>n;

    for(int i=1;i<=n;i++)
        in>>a[i].x>>a[i].y;

    a[n+1]=a[1];

    for(int i=1;i<=n;i++)
        arie+=Aria(a[i],a[i+1]);

    out<<setprecision(7)<<arie;

    return 0;
}