#include <bits/stdc++.h>
#define NR 100005
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
int i,j,n,m;
double x[NR], y[NR], aria;
int main()
{
    fin>>n;
    for(i=1;i<=n;++i){
        fin>>x[i]>>y[i];
    }
    y[n+1]=y[1];
    x[n+1]=x[1];
    for(i=1;i<=n;++i){
        aria = aria + (x[i]*y[i+1]-x[i+1]*y[i]);
    }
    fout<<fixed<<setprecision(6)<<aria/2<<'\n';
    return 0;
}