#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
struct{
int x,y;}P[100003];
int main()
{
    int n;
    double  s= 0;
    in>>n;
    for(int i = 1 ; i<= n ; i++)
        in>>P[i].x>>P[i].y;
    P[n+1] = P[1];
    n++;
    for(int i = 1 ; i < n ; i++)
        s += (P[i].x*P[i+1].y - P[i+1].x*P[i].y);
    s /= 2.0;
    out<<setprecision(5)<<fixed<<s;


    return 0;
}