#include <bits/stdc++.h>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");

long double arie(pair<long double, long double> a, pair<long double, long double> b){
    return (a.first-b.first)*(a.second+b.second);
}
int main(){
    int n;
    fin>>n;
    vector<pair<long double,long double> > v(n);
    for(int i=0; i<n; i++){
        fin>>v[i].first>>v[i].second;
    }
    long double area=0;
    for(int i=0; i<n-1; i++){
        area+=arie(v[i], v[i+1]);
    }
    area+=arie(v[n-1], v[0]);
    if(area<0){
        area=-area;
    }
    fout<<setprecision(6)<<fixed;
    fout<<area/2;
}