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

ifstream f("aria.in");
ofstream g("aria.out");

long long x[100001],y[100001],sol;
int n;

int main()
{
    f>>n;
    for(int i=1;i<=n;++i)
    {
        f>>x[i]>>y[i];
    }
    for(int i=1;i<=n;++i)
        sol+=(x[i]*y[i%n+1])-(y[i]*x[i%n+1]);
        sol=abs(sol);
    g<<fixed<<setprecision(5)<<double(sol)/2.0;
}