#include<stdio.h>
#include<math.h>
int main () {
	freopen("aria.in","r",stdin);
	freopen("aria.out","w",stdout);
	int N,i;
	float X[1000001],Y[100001],A=0;
	scanf("%d",&N);
	for(i=1;i<=N;i++)
		scanf("%f %f",&X[i],&Y[i]);
		X[N+1]=X[1];
		Y[N+1]=Y[1];
	for(i=1;i<=N;i++)
		A=A+(X[i]*Y[i+1]-X[i+1]*Y[i]);
	A=A/2;
	printf("%.5f",fabs(A));
	fclose(stdin);
	fclose(stdout);
	return 0;
	}