#include <bits/stdc++.h>
using namespace std;
ifstream fin ("aria.in");
ofstream fout ("aria.out");
int n,i;
long double xx,yy,Arie;
long double arie(long double x1,long double y1,long double x2,long double y2){
        return (x1-xx)*(y2-yy)-(x2-xx)*(y1-yy);
}
pair<long double,long double> v[100010];
int main(){
    fin>>n;
    for(i=1;i<=n;i++)
        fin>>v[i].first>>v[i].second;
    xx=v[1].first;
    yy=v[1].second;
    for(i=2;i<=n-1;i++){
        Arie+=arie(v[i].first,v[i].second,v[i+1].first,v[i+1].second);
    }
    fout<<setprecision(7)<<fixed<<Arie/2;
    return 0;
}