#include "stdio.h"

FILE *f, *g;
int N;
double x[100000], y[100000];
double sum;

int main()
{
	f = fopen("aria.in", "r");
	g = fopen("aria.out", "w");

	sum = 0;

	fscanf(f, "%d", &N);

	for(int i = 0; i < N; i++)
	{
		fscanf(f, "%lf", &x[i]);
		fscanf(f, "%lf", &y[i]);
	}

	for(int i = 0; i <= N-2; i++)
		sum = sum + (x[i] * y[i+1] - x[i+1] * y[i]);
	sum = sum + x[N-1] * y[0] - x[0] * y[N-1];
	sum = sum / 2.0;
	
	fprintf(g, "%.5lf", sum);

	fclose(f);
	fclose(g);

	return 0;
}