#include<stdio.h>
#include<math.h>

#include<iostream>
#include<fstream>

using namespace std;

#define MAXN 100000

int N;

int main(){
	
	freopen("aria.in", "r", stdin);
	freopen("aria.out", "w", stdout);

	int N;
	scanf("%d",&N);

	double x0, y0, x, y, x1, y1; 
	scanf("%lf %lf", &x, &y);
	x0=x; y0=y;

	double A=0;
	for(int i=1;i<N;i++){
		scanf("%lf %lf", &x1, &y1);
		A+=((x*y1-x1*y)/2);
		x=x1; y=y1;
	}

	A+=((x*y0-x0*y)/2);
	printf("%.5f\n",abs(A));

	return 0;
}