#include<bits/stdc++.h>
using namespace std;

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

const int NMAX=100005;

int n;
double a[NMAX],b[NMAX];
double sol;

int main()
{
    int i;
    fin>>n;sol=0;
    for (i=1;i<=n;i++) fin>>a[i]>>b[i];
    a[0]=a[n];b[0]=b[n];
    for (i=0;i<n;i++)
        sol=sol+(a[i]*b[i+1]-a[i+1]*b[i]);
    if (sol<0) sol*=-1;
    sol=sol/2.0;
    fout<<sol<<"\n";
    return 0;
}