#include <fstream>
using namespace std;
ifstream cin("aria.in");
ofstream cout("aria.out");
int n;
float x[100001],y[100001],t[100001];
int main()
{int i;
    cin>>n;
    for (i=1;i<=n;i++)
      cin>>x[i]>>y[i];
    n++;
    x[n]=x[1];
    y[n]=y[1];
    for (i=1;i<n;i++)
      t[i]=(x[i]*y[i+1]-x[i+1]*y[i])/2;
float s=0;
    for (i=1;i<n;i++) s+=t[i];
    cout<<s;
    cin.close();
    cout.close();
    return 0;
}