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

int v1[100001],v2[100001];
int main(){
  FILE *fin,*fout;
  long long int n,a,b,p,i,ca;

  fin=fopen("aria.in", "r");
  fout=fopen("aria.out", "w");
  fscanf(fin, "%lld", &n);
  for(i=0;i<n;i++)
    fscanf(fin, "%lld%lld", &v1[i],&v2[i]);
  v1[n]=v1[0];
  v2[n]=v2[0];
  p=0;
  for(i=0;i<n;i++)
    p=p+(v1[i+1]-v1[i])*(v2[i]+v2[i+1]);
  if(p<0)
    p=-p;
  fclose(fin);

  if(p%2==0)
    fprintf(fout, "%lld\n", p/2);
  else
    fprintf(fout, "%lld.500000\n", p/2);
  fclose(fout);
  return 0;
}