#include <bits/stdc++.h>

using namespace std;

struct node
{
    double x,y;
} vec[100005];
int n;
double sol;

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


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