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

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");
const int de = 100005;

struct point
{
    double x, y;
};

double arie(point A, point B, point C) {
    A.x -= C.x;
    A.y -= C.y;
    B.x -= C.x;
    B.y -= C.y;
    return ((A.x * B.y) - (A.y * B.x)) / 2;
}

double arie_cv(int n, point p[] ){
    double tot = 0;
    for( int i =  2 ; i < n ; ++i ){
        tot += arie(p[1], p[i], p[i+1]);
    }
    return abs(tot);
}


int main(){
    
    int n;
    point p[de];
    fin >> n;
    
    for( int i = 1 ; i <= n ; ++i ){
        fin >> p[i].x >> p[i].y;
    }
    
    fout << arie_cv(n, p);
    
    
    return 0;
}