#include<cstdio>
#include<cmath>
#define N_MAX 100000
using namespace std;

struct point
{
    double x, y;
};

point p[N_MAX];
int n;
double sum;

int main()
{
    int i;
    FILE *fin, *fout;
    fin = fopen("aria.in","r");
    fout = fopen("aria.out","w");
    fscanf(fin,"%d",&n);
    for(i=0; i<n; i++)
        fscanf(fin,"%lf%lf",&p[i].x,&p[i].y);
    p[n] = p[0];
    for(i=0; i<n; i++)
        sum += (p[i].x*p[i+1].y - p[i+1].x*p[i].y);
    fprintf(fout,"%.5f\n",fabs(sum / 2.0));
    fclose(fin);
    fclose(fout);
    return 0;
}