#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

struct point{
    long double x,y;
}puncte[100005];

int n;

long double calc_arie(point a, point b){
    return a.x*b.y-b.x*a.y;
}
long double arie=0;
void citire(){
    f>>n;
    f>>puncte[0].x>>puncte[0].y;
    for(int i=1; i<n; i++){
        f>>puncte[i].x>>puncte[i].y;
        arie+=calc_arie(puncte[i-1],puncte[i]);
    }
    arie+=calc_arie(puncte[n-1],puncte[0]);
    arie/=2;
    g<<setprecision(6)<<fixed<<arie<<endl;
}


int main() {
    citire();
    return 0;
}