#include<stdio.h>
#include<stdlib.h>
int main(){
FILE *fin, *fout;
fin = fopen ("aria.in", "r");
fout = fopen ("aria.out", "w");
 int N, x1, y1, x2, y2, i, xi, yi;
int  Arie, Ai;
 Arie = 0;
 Ai = 0;
 fscanf(fin, "%d", &N);
 fscanf(fin, "%d%d", &x1, &y1);
 xi = x1;
 yi = y1;
for(i = 1; i < N; i++)
{ fscanf(fin, "%d%d", &x2, &y2);
  Ai = (x1 * y2 - x2 * y1) / 2;
 //printf("%lf", Ai);
  if( Ai > 0)
  Arie = (Arie + Ai) ;
  else
  Arie = (Arie - Ai) ;
  x1 = x2;
  y1 = y2;
}
Ai = (x1 * yi - xi * y1) / 2;
 if( Ai > 0)
  Arie = (Arie + Ai) ;
  else
  Arie = (Arie - Ai) ;
fprintf(fout, "%d", Arie);
 return 0;
}