#include <bits/stdc++.h>

using namespace std;

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

struct punct
{
    long double x,y;
};
punct a[100001];
int n;
long double arie;

long double triArie(punct a,punct b,punct c)
{
    return fabs( a.x*(b.y-c.y)+b.x*(c.y-a.y)+c.x*(a.y-b.y) )*0.5;
}

int main()
{
    in>>n;

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

    for(int i=2;i<n;i++)
        arie+=triArie(a[1],a[i],a[i+1]);

    out<<arie;

    return 0;
}