#include<bits/stdc++.h>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int n;
double S;
struct points
{
    double x,y;
};
points v[100005];
int main()
{
    f>>n;
    for(int i=1;i<=n;++i)
        f>>v[i].x>>v[i].y;
    v[n+1].x=v[1].x;
    v[n+1].y=v[1].y;
    for(int i=1;i<=n;++i)
        S=S+0.500000*(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    g<<fixed<<setprecision(5)<<S;
    return 0;
}