#include <iostream>
#include <fstream>
#include <iomanip>
#define LL long long
#define DN 100005
#define x first
#define y second
using namespace std;

typedef pair<int,int> per;

int n;
per p[DN];

LL det(per a,per b, per c) {
    return a.x*b.y+b.x*c.y+c.x*a.y-a.x*c.y-b.x*a.y-c.x*b.y;
}

int main() {
    ifstream f("aria.in");
    ofstream g("aria.out");
    f>>n;
    for(int i=1; i<=n; ++i) f>>p[i].x>>p[i].y;
    double ar=0;
    for(int i=2; i<n; ++i) ar+=det(p[1],p[i],p[i+1]);
    g<<fixed<<setprecision(6)<<ar*0.5;
}