#include <cstdio>
struct aa{double x,y;};
aa v[100010];
double arie(aa a,aa b,aa c)
    {
    return (a.x-b.x)*(a.y-c.y)-(a.x-c.x)*(a.y-b.y);
    }
int main()
{freopen("aria.in", "r", stdin);
freopen("aria.out", "w", stdout);
int n,i;
double sol;
scanf("%d",&n);
for(i=1;i<=n;i++)
    scanf("%lf%lf",&v[i].x,&v[i].y);
v[n+1]=v[1];
sol=0;
for(i=1;i<=n;i++)
    sol+=arie(v[0],v[i],v[i+1]);
printf("%.5lf",sol/2);
return 0;
}