#include <stdio.h>
#include<stdlib.h>
long double modul(long double a)
{
	if (a < 0)
	{
		return -a;
	}
	else
	{
		return a;
	}
}
long double ploshad_treugolnika(long XA, long YA, long XB, long YB, long XC, long YC)
{
	long double S = modul((XB - XA) * (YC - YA) - (XC - XA) * (YB - YA))/2;
	return S;
}
void function(long* array, long size) {
	long double SUM = 0;
	for (long i = 5; i < size; i += 2) {
		long double S = ploshad_treugolnika(array[0], array[1], array[i - 3], array[i - 2], array[i - 1], array[i]);
		SUM = (SUM + S);
	}
	printf("%3.10Lg", SUM);
}
int main()
{
	long N, x, y;
	scanf("%ld", &N);
	long* pa;
	pa = (long*)malloc(2 * N * sizeof(long));
	for (int q = 0; q < 2 * N; q += 2) {
		scanf("%ld%ld", &x, &y);
		pa[q] = x;
		pa[q + 1] = y;
	}
	function(pa, 2 * N);
	free(pa);
	return 0;
}