#include <bits/stdc++.h>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
const int N = 100010;
double x[N],y[N],aria;
int n;
int main()
{   f>>n;
    for(int i=1;i<=n;i++)
        f>>x[i]>>y[i];
    x[n+1]=x[1];
    y[n+1]=y[1];
    for(int i=1;i<n;i++)
        aria+=x[i]*y[i+1]-x[i+1]*y[i];
    if(aria<0)aria=-aria;
    aria/=2.0;
    g<<fixed<<setprecision(10)<<aria<<'\n';
    return 0;
}