#include <fstream>
#define x first
#define y second
using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

int n,i,j;
pair <int,int> v[100001];
double sol;

int det(pair<int,int> a, pair<int,int> b){
    return a.x*b.y-a.y*b.x;
}

int main(){
    fin>>n>>v[1].x>>v[1].y;
    for(i=2;i<=n;i++){
        fin>>v[i].x>>v[i].y;
        sol+=det(v[i-1],v[i])/2.0;
    }
    sol+=det(v[n],v[1])/2.0;

    fout<<max(sol,-sol);

    return 0;
}