#include <bits/stdc++.h>

#include <iomanip>

using namespace std;

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

int n;
long double x[100001],y[100001];

void Citire(){
    fin >> n;
    for(int i = 1 ; i <= n ; i++)
        fin >> x[i] >> y[i];
}

long double Arie(long double x[], long double y[], int n){
    long double s = 0.0;
    x[n+1] = x[1];
    y[n+1] = y[1];
    for(int i = 1 ; i <= n ; i++)
        s += abs(x[i]*y[i+1] - x[i+1]*y[i]);
    return s/(2.0);
}

int main()
{
    Citire();
    long double s = 0;
    x[n+1] = x[1];
    y[n+1] = y[1];
    for(int i = 1 ; i <= n ; i++)
        s = s + (x[i]*y[i+1] - x[i+1]*y[i]);
   fout << setprecision(9) << s/2;
    return 0;
}