#include <stdio.h>
#include <string.h>
#include <algorithm>

using namespace std;

const int Mn = 1e5 + 6;

int n;
double sol;
double x[Mn],y[Mn];

int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);

     scanf("%d",&n);

     for (int i = 1;i <= n;++i)
        scanf("%lf %lf",&x[i],&y[i]);

     x[n + 1] = x[1];
     y[n + 1] = y[1];

     for (int i = 1;i <= n;i++)
         sol += x[i] * y[i + 1] - x[i + 1] * y[i];

     printf("%1.12f\n",sol / 2);

  return 0;
}