#include <cstdio>
using namespace std;
int n;
double aria,x0,y0,x1,y1,x2,y2;
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&n);
    scanf("%lf%lf",&x0,&y0);
    scanf("%lf%lf",&x1,&y1);
    for(n-=2;n;n--)
    {
        scanf("%lf%lf",&x2,&y2);
        aria+=x0*y1+x1*y2+x2*y0-y0*x1-y1*x2-y2*x0;
        x1=x2;y1=y2;
    }
    if(aria<0)aria=-aria;
    printf("%lf",aria/2.0);
    return 0;
}