#include <iostream>
#include <fstream>
using namespace std;
ifstream in("tri.in");
ofstream out("tri.out");
int main()
{

double a=0,x[100],y[100];
int i,N;
    in>>N;
    for(i=1; i<=N; i++)
    {
        in>>x[i]>>y[i];
    }
    for(i=1; i<=N; i++)
    {
        a+=(x[i]*y[i+1]-x[i+1]*y[i]);
    }
    a+=x[N]*y[1]-x[1]*y[N];
    out <<a/2;
    return 0;
}