#include <fstream>
#include <cstdio>
using namespace std;

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

struct point{
    double x, y;
}v[100001];

double s;
int n;

void read()
{
    f>>n;
    for(int i=0; i<n; ++i) f>>v[i].x>>v[i].y;
    v[n].x=v[0].x;
    v[n].y=v[0].y;
    v[n+1].x=v[1].x;
    v[n+1].y=v[1].y;
}

void solve()
{
    int i;
    for(i=1; i<=n; ++i)
        s += v[i].x*(v[i+1].y-v[i-1].y);
}
int main()
{
    freopen("aria.out", "w", stdout);
    read();
    solve();
    printf("%.4f", s/2.0);
}