#include <iostream>
#include<cstdio>
using namespace std;

int main()
{
    int n,i;
    FILE*fin,*fout;
    double s=0,x1,x2,y1,y2;
    fin=fopen("aria.in","r");
    fout=fopen("aria.out","w");
    fscanf(fin,"%d%lf%lf",&n,&x1,&y1);
    for(i=2;i<=n;i++){
      fscanf(fin,"%lf%lf",&x2,&y2);
      s+=(x1-x2)*(y1+y2);
      x1=x2;
      y1=y2;
    }
    s/=2.0;
    fprintf(fout,"%.5lf",s);
    return 0;
}