#include <bits/stdc++.h>

using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
const int L = 1e5 + 5;
struct C
{
   long double x, y;
};
C p[L];
int main()
{
    long double n, s = 0;
    f >> n;
    for(int i = 1; i <= n; i++)
        f >> p[i].x >> p[i].y;
    for(int i = 1; i <= n; i++)
    {
        if(i == n)
        {
            p[i + 1].x = p[1].x;
            p[i + 1].y = p[1].y;
        }
        s += p[i].x * p[i + 1].y - p[i + 1].x * p[i].y;
    }
    g << fixed  << setprecision(6) << (long double)s / (long double)2;
    return 0;
}