#include <iostream>
#include <fstream>
#include <cmath>
#include <iomanip>

#define maxN 100005

using namespace std;

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

struct interval {int x,y;} v[maxN];
int n;
long long S;

int main()
{
    fin>>n;
    for (int i=1; i<=n; ++i)
        fin>>v[i].x>>v[i].y;
    v[n+1]=v[1];
    for (int i=1; i<=n; ++i)
        S+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    fout<<fixed<<setprecision(5)<<S/2;
    return 0;
}