#include<stdio.h>

#define maxn 100005

FILE*f=fopen("aria.in","r");
FILE*g=fopen("aria.out","w");

int n;
double x[maxn],y[maxn];

int main () {
	
	fscanf(f,"%d",&n);
	for ( int i = 1 ; i <= n ; ++i ){
		fscanf(f,"%lf %lf",&x[i],&y[i]);
	}
	
	double A = 0;
	x[n+1] = x[1],y[n+1] = y[1];
	for ( int i = 1 ; i <= n ; ++i ){
		A += (1LL*x[i]*y[i+1]) - (1LL*x[i+1]*y[i]);
	}
	
	if ( A < 0 )	A = -A;
	A /= 2;
	fprintf(g,"%lf\n",A);
	
	fclose(f);
	fclose(g);
	
	return 0;
}