#include <cstdio>
#include <algorithm>
#include <cmath>
using namespace std;
FILE*fin=fopen("aria.in","r");
FILE*fout=fopen("aria.out","w");
int i,j,k,a,b,rasp,raspe,nr,nre,n;
double sum;
struct alfa {int x;int y;};
alfa v[1100000];
int main()
{
    fscanf(fin,"%d",&n);
    for (i=0;i<n;i++)
    fscanf(fin,"%d %d",&v[i].x,&v[i].y);

    v[n].x=v[1].x;
    v[n].y=v[1].y;

    for (i=0;i<=n;i++)
    sum+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);

fprintf(fout,"%lf",abs(sum/2));
    return 0;
}