#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#define min(p,q)((p)<(q)?(p):(q))
#define zahyoutype long

typedef struct Point{zahyoutype x,y;}P;
zahyoutype crossproduct(P p,P q,P o){return (p.x-o.x)*(q.y-o.y)-(p.y-o.y)*(q.x-o.x);}
void readpoint(P*p){zahyoutype x=1;scanf(x/2?"%lf%lf":"%ld%ld",&p->x,&p->y);}

int main(){
	int q;
	scanf("%d",&q);
	P o,p1,p2;
	readpoint(&o);
	readpoint(&p1);
	q-=2;
	zahyoutype s=0;
	while(q--){
		readpoint(&p2);
		s+=crossproduct(p1,p2,o);
		p1=p2;
	}
	printf("%.1f\n",fabs(s/2.0));
}
