#include<stdio.h> 
#include<stdlib.h>
FILE * Fin=fopen("aria.in","r"), *Fout=fopen("aria.out","w");
int main ()
{
    double aria=0,a[100001][2];
	int i,j,n;
    fscanf(Fin,"%d",&n);
    for(i=0;i<n;++i)
        for(j=0;j<2;++j)
            fscanf(Fin,"%lf",&a[i][j]);
	for(i=0;i<n-1;++i)
	{
        aria+=(a[i][0]*a[i+1][1]-a[i][1]*a[i+1][0])/2;
    }
	aria+=(a[n-1][0]*a[0][1]-a[n-1][1]*a[0][0])/2;
    fprintf(Fout,"%lf",aria);
    return 0;
}