#include<stdio.h>
#include<assert.h>
#include<math.h>
#define MAX 100005
int main () {
	int N,i;
	double X[MAX],Y[MAX],A=0;
	assert(freopen("aria.in","r",stdin));
	assert(freopen("aria.out","w",stdout));
	assert(scanf("%d",&N)==1);
	assert(1<=N && N<=100000);
	for(i=0;i<N;i++) {
		assert(scanf("%lf%lf",&X[i],&Y[i])==2);
		assert(-10000000<=X[i] && X[i]<=10000000);
		assert(-10000000<=Y[i] && Y[i]<=10000000);
		}
		X[N]=X[0];
		Y[N]=Y[0];
	for(i=0;i<N;i++)
		A=A+(X[i]*Y[i+1]-X[i+1]*Y[i]);
		A=A/2;
	printf("%.5lf",fabs(A));
	fclose(stdout);
	fclose(stdin);
	return 0;
	}