#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,X,Y;
double sum;
struct alfa {double x;double y;};
alfa v[1100000];
int main()
{
    fscanf(fin,"%d",&n);
    for (i=0;i<n;++i)
    fscanf(fin,"%lf %lf",&v[i].x,&v[i].y);
    v[n].x=v[0].x;
    v[n].y=v[0].y;
    for (i=0;i<n;i++)
    sum+=double((double)v[i].x*v[i+1].y-(double)v[i+1].x*v[i].y);
    sum=(double)sum/2;
fprintf(fout,"%lf",abs(sum));
    return 0;
}