#include<iostream>
#include<iomanip>
#include<fstream>
using namespace std;

int main(){
    ifstream cinr("aria.in");
    int n; cinr >> n;
    double a,b,x,y,s=0,f,k;
    cinr >> a >> b; f=a; k=b;
    for(int i=1; i<n; i++){
            cinr >> x >> y;
            s+=0.5*(a*y-b*x);
            a=x; b=y;
            }
    s+=0.5*(x*k-y*f);
    ofstream cour("aria.out");
    cour << setprecision(8) << s;
    return 0;
    }