#include <bits/stdc++.h>

using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
const int N = 100001;
int n;
long double a,x,y;
complex<long double > z[N];
int main()
{
    f>>n;
    for(int i=1;i<=n;i++)
    {
        f>>x>>y;
        z[i]=complex<long double>(x,y);
    }
    z[0]=z[n];
    for(int i=0;i<n;i++)
        a+=imag(z[i]*conj(z[i+1]));
    if(a<0)
        a=-a;
    g<<fixed<<setprecision(10)<<a/2.0;
    return 0;
}