#include<cstdio>
struct aa{int x,y;};
aa v[100001];
int main ()
{freopen ("aria.in","r",stdin);
freopen ("aria.out","w",stdout);
int n,i;
double s;
scanf("%d",&n);
for(i=1;i<=n;i++)
    scanf("%d%d",&v[i].x,&v[i].y);
v[n+1]=v[1];
s=0.0;
for(i=1;i<=n;i++)
    s+=v[i].x*v[i+1].y-v[i+1].x*v[i].y;
s=s*0.5;
if(s<0)
    s=-s;
printf("%.5lf",s);
return 0;
}