#include <iostream>
#include <fstream>
#include <iomanip>
#define DIM 100002
#define pr pair<long double, long double>
#define x first
#define y second

using namespace std;

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

int n;

long double A;

pr p[DIM], pctar;

long double det(pr a, pr b){
    return (long double)(a.x * b.y - b.x * a.y) / (long double)2.0;
}

int main() {
    in>>n;
    for(int i = 1; i <= n; ++ i)
        in>>p[i].x>>p[i].y;
    pctar = make_pair(0, 0);
    p[n + 1] = p[1];
    p[n + 2] = p[2];
    for(int i = 1; i <= n; ++ i){
        A += det(p[i], p[i + 1]);
    }
    out<<setprecision(7)<<fixed<<(long double)(A);
    return 0;
}