#include <stdio.h>
#include <stdlib.h>
#include<math.h>
int main()
{
    FILE *fisier,*fisier1;
    fisier=fopen("aria.in","r");
    fisier1=fopen("aria.out","w");
    int x[100001],y[100001],i,n;
    float aria=0,s;
    fscanf(fisier,"%d",&n);
    for(i=1;i<=n;i++)
        fscanf(fisier,"%d%d",&x[i],&y[i]);
    for(i=2;i<n;i++) {
        s=fabs(x[1]*y[i]+x[i+1]*y[1]+x[i]*y[i+1]-x[i+1]*y[i]-x[1]*y[i+1]-x[i]*y[1]);
        aria=aria+s/2;
    }
    fprintf(fisier1,"%f",aria);
    fclose(fisier);
    fclose(fisier1);
    return 0;
}