#include <iostream>
#include<cstdio>

using namespace std;

int main()
{
    int n,i;
    FILE*fin,*fout;
    double s=0,x1,x2,y1,y2,x3,y3;
    fin=fopen("aria.in","r");
    fout=fopen("aria.out","w");
    fscanf(fin,"%d%lf%lf",&n,&x1,&y1);
    x3=x1;
    y3=y1;
    for(i=2;i<=n;i++){
      fscanf(fin,"%lf%lf",&x2,&y2);
      s+=(x1-x2)*(y1+y2);
      x1=x2;
      y1=y2;
    }
    s+=(x2-x3)*(y3+y2);
    s/=2;
    fprintf(fout,"%.6lf",s);
    return 0;
}