#include<bits/stdc++.h>
#define maxN 100005
using namespace std;
int n,r,l;
double x,y;
pair<double,double> v[maxN];
double aria;
int main()
{
   // freopen("aria.in","r",stdin);
   // freopen("aria.out","w",stdout);
    ifstream fin("aria.in");
    ofstream fout("aria.out");
    fin>>n;
    for(int i=1;i<=n;i++)
    {
    //    scanf("%d%d",&x,&y);
        fin>>x>>y;
        v[i]={x,y};
    }

    for(int i=1;i<=n;i++)
    {
        r=i+1;
        if(r>n) r=1;
        l=i;
        aria=aria+(double)((v[l].first*v[r].second-v[r].first*v[l].second)/2.0);
    }

    fout<<setprecision(6)<<fixed<<aria<<'\n';

    return 0;
}