#include <bits/stdc++.h>

using namespace std;

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

struct pct
{
    long double x, y;
}v[100005];
int n, i;
long double aria;

int main()
{
    f >> n;
    for(i = 1; i <= n; ++ i)
        f >> v[i].x >> v[i].y;
    v[++ n].x = v[1].x;
    v[n].y = v[1].y;
    for(i = 1; i < n; ++ i)
        aria = aria + (v[i].x * v[i + 1].y - v[i + 1].x * v[i].y);
    g << fixed << setprecision(6) << fabs(aria / 2.0);
    return 0;
}