#include<cstdio>
#include<cmath>

using namespace std;

const char InFile[]="aria.in";
const char OutFile[]="aria.out";
const int MaxN=100010;

int N;
struct punct
{
	double x,y;
}P[MaxN];
	
double solve()
{
	double sol=0;
	int i;
	for( i=0 ; i<N ; i++ )
		sol += ( P[i].x*P[i+1].y - P[i+1].x*P[i].y );
	return fabs(sol/2);
}

int main()
{
	FILE *fin,*fout;
	fin=fopen(InFile,"rt");
	fout=fopen(OutFile,"wt");
	fscanf(fin,"%d", &N);
	for( int i=0 ; i<N ; i++ )
		fscanf(fin,"%lf%lf", &P[i].x, &P[i].y);
	P[N]=P[0];
	fprintf( fout, "%lf" , solve() );
	return 0;
}