#include <bits/stdc++.h>

using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
const int N = 100001;
int n;
double a,x[N],y[N];
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=0;i<n;i++)
    {
        a+=(x[i]-x[i+1])*(y[i]+y[i+1])/2.0;
    }
    g<<a;
    return 0;
}