/*
    Keep It Simple!
*/

#include<stdio.h>
int n;
double x[100005],y[100005];
double A;


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

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

    x[0] = x[n];
    y[0] = y[n];

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

    if( A < 0 ) A = -A;
    A/=2;

    printf("%.5f ",A);

}