#include<stdio.h>
#include<math.h>
using namespace std;

#define MAX 100002

FILE *in, *out;

int N;
struct POINT
{
	double x, y;
};

POINT  P[MAX];


int main()
{
	
	double A = 0;
	in = fopen("aria.in", "r");
	out = fopen("aria.out", "w");
	fscanf(in,"%d", &N);
	for (int i = 1;i <= N;++i)
	{
		fscanf(in, "%lf%lf", &P[i].x,&P[i].y);
	}
	P[N + 1] = P[1];
	for (int i = 1;i <= N;++i)
	{
		A += (P[i].x*P[i+1].y - P[i+1].x*P[i].y);
	}
	fprintf(out, "%.6lf", fabs(A / 2));
	return 0;
}