#include <stdio.h>
#include <stdlib.h>

struct poligon{
  double x, y;
} v[100001];

int main()
{
    FILE *fin, *fout;
    int n, i;
    double arie;
    fin=fopen("aria.in", "r");
    fscanf(fin, "%d", &n);
    for(i=0; i<n; i++)
      fscanf(fin, "%lf%lf", &v[i].x, &v[i].y);
    fclose(fin);
    v[n]=v[0];
    arie=0.0;
    for(i=0; i<n; i++)
      arie+=v[i].x*v[i+1].y-v[i+1].x*v[i].y;
    fout=fopen("aria.out", "w");
    fprintf(fout, "%.6lf", arie/2.0);
    fclose(fout);
    return 0;
}