#include <stdio.h>
#define MAX 100005

int n, i;
long double x[MAX], y[MAX], A;

int main(){
	freopen("aria.in", "r", stdin);
	freopen("aria.out", "w", stdout);
	scanf("%d", &n);
	for(i = 0; i < n; i++)
		scanf("%Lf%Lf", &x[i], &y[i]);
	x[n] = x[0], y[n] = y[0];
	for(i = 0; i < n; i++)
		A += (x[i] * y[i + 1] - x[i + 1] * y[i]) / 2;
	printf("%.5Lf\n", A);
	return 0;
}