#include <bits/stdc++.h>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
const int N = 100010;
int n;
long double x[N],y[N],aria;
long double det(int i,int j,int k)
{
    return (x[i]*y[j]+x[j]*y[k]+x[k]*y[i])-(x[i]*y[k]+x[k]*y[j]+x[j]*y[i]);
}
int main()
{
    f >> n ;
    for(int i = 1;i <= n;i++)
        f >> x[i] >> y[i];
    x[0] = x[n];
    y[0] = y[n];
    for(int i=2;i<n;i++)
        aria += det(1,i,i+1)/2;
    g<<fixed<<setprecision(6)<<aria;
    return 0;
}