#include <stdio.h>
using namespace std;
FILE *f=fopen("aria.in","r"),*g=fopen("aria.out","w");
int n,i;
double x[100004],y[100004],aria;
int main(void)
{
    fscanf(f,"%d",&n);
    for (i=1;i<=n;i++)
      fscanf(f,"%lf%lf",&x[i],&y[i]);
    x[n+1]=x[1];
    y[n+1]=y[1];
    for (i=1;i<=n;i++)
        aria=aria+x[i]*y[i+1]-x[i+1]*y[i];
    aria/=2;
    if (aria<0) aria*=-1;
    fprintf(g,"%.5lf",aria);
    return 0;
}