#include <iostream>
#include <fstream>
#include <math.h>
#include<iomanip>
using namespace std;

int main(){
	ifstream fin("aria.in");
	ofstream fout("aria.out");
	int n, x, y, A = 0 , vx[1000000], vy[1000000];
	fin >> n;
	for(int i = 0; i < n; i++){
		fin >> x >> y;
		vx[i] = x;
		vy[i] = y;

}
    vx[n] = vx[0];
    vy[n] = vy[0];
    for(int j = 0; j < n; j++){
    	A += ((vx[j]*vy[j+1])-(vx[j+1]*vy[j]))/2;
    }
    fout << fixed << setprecision (7)<<A'\n';
    return 0;
}