#include <bits/stdc++.h>

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

struct punct
{
    double x, y;
}p[100005];
int n;

int main()
{
    fin>>n;
    for(int i=1; i<=n; ++i) fin>>p[i].x>>p[i].y;
    p[n+1]=p[1];
    double s=0;
    for(int i=1; i<=n; ++i)
    {
        s=s+p[i].x*p[i+1].y-p[i].y*p[i+1].x;
    }
    if(s<0) s=s*(-1);
    s=s/2;
    fout<<fixed<<setprecision(5)<<s<<"\n";
    return 0;
}