#include <bits/stdc++.h>
#define NM 100005
using namespace std;
ifstream fin ("aria.in");
ofstream fout ("aria.out");
struct punct
{
    int x, y;
};
int n;
long double aria;
punct v[NM];
int cross(punct a, punct b)
{
    return a.x*b.y - a.y*b.x;
}
int main()
{
    fin >> n;
    for(int i=1; i<=n; i++)
        fin >> v[i].x >> v[i].y;
    v[n+1] = v[1];
    for(int i=1; i<=n; i++)
        aria+=cross(v[i], v[i+1]);
    aria/=2;
    fout << setprecision(5) << fixed << aria;


    return 0;
}