#include <iostream>
#include <fstream>

using namespace std;

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

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

void Citire(){
    fin >> n;
    for(int i = 0 ; i < n ; i++)
        fin >> x[i] >> y[i];
}

long double Arie(double x[], double y[], int n){
    long double s = 0.0;
    int j = n - 1;
    for(int i = 0 ; i < n ; i++){
        s += (x[j]+x[i])*(y[j]-y[i]);
        j = i;
    }
    if(s < 0)
        return (-s)/(2.0);
    else
        return s/(2.0);
}

int main()
{
    Citire();
    fout << Arie(x,y,n);
    return 0;
}