#include<stdio.h>
#include<utility>
#include<algorithm>
#include<math.h>
using namespace std;
FILE *in,*out;
int n,i;
long double final,sol
pair<int, int> coord[100001];
int main()
{
    in=fopen("aria.in","rt");
    out=fopen("aria.out","wt");
    fscanf(in,"%d",&n);
    for(i=1;i<=n;i++)
        fscanf(in,"%d%d",&coord[i].first,&coord[i].second);
    coord[n+1]=coord[1];
    for(i=1;i<=n;i++)
        sol+=(coord[i].first*coord[i+1].second-coord[i+1].first*coord[i].second);
    final=abs(sol)/2;
    fprintf(out,"%.10llf",final);
    fprintf(out,"\n");
    fclose(in);
    fclose(out);
    return 0;
}