#include <stdio.h>          // printf(), scanf()
#define MAX_N 100

int
main(int argc, char** argv)
{
	int x[MAX_N], y[MAX_N];
	int n;
	int i;

	scanf("%d", &n);
	for (i = 0; i < n; ++i)
		scanf("%d %d", &x[i], &y[i]);

	double area = 0.0;
	for (i = 1; i <= n; ++i)
		area += x[i - 1] * y[i % n] - y[i - 1] * x[i % n];

	area /= 2;
	printf("%.1f\n", area);
	return 0;
}