#include <bits/stdc++.h>

using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
struct punct
{
    double x, y;
}a[100010];
int n;
double aria;
int main()
{
    f>>n;
    for(int i=1; i<=n; i++)
        f>>a[i].x>>a[i].y;
    for(int i=1; i<n; i++)
        aria+=a[i].x*a[i+1].y-a[i+1].x*a[i].y;
    aria+=a[n].x*a[1].y-a[1].x*a[n].y;
    aria/=2;
    g<<setprecision(6)<<aria;
    return 0;
}