#include<stdio.h>
#define MAXN 100002

struct punct
{
	double x, y;
}vector[MAXN];

int n;
double aria=0;

int main()
{
	FILE *inputFile=fopen("aria.in", "r"), *outputFile=fopen("aria.out", "w");
	fscanf(inputFile, "%d", &n);
	for(int i=0; i<n; i++)
	{
		fscanf(inputFile, "%lf %lf", &vector[i].x, &vector[i].y);
	}
	vector[n]=vector[0];
	for(int i=0; i<n; i++)
	{
		aria+=(vector[i].x * vector[i+1].y - vector[i+1].x * vector[i].y);
	}
	fprintf(outputFile, "%lf", aria/2);

	return 0;
}