#include <bits/stdc++.h>

using namespace std;

struct node
{
    double x,y;
};
int n;
double area;
node a[100001];

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

void Read()
{
    f>>n;
    for(int i=0;i<n;++i)
        f>>a[i].x>>a[i].y;
    a[n] = a[0];
    f.close();
}

void Solve()
{
    for (int i = 0; i < n; ++i)
        area += (a[i].x * a[i + 1].y - a[i + 1].x * a[i].y);
}

void Print()
{
    g <<(area / 2.0);
    g.close();
}

int main()
{
    Read();
    Solve();
    Print();
    return 0;
}