#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);
    x1=x0;y1=y0;
    for(n--;n;n--)
    {
        scanf("%lf%lf",&x2,&y2);
        aria+=x1*y2-x2*y1;
        x1=x2;y1=y2;
    }
    x2=x0;y2=y0;
    aria+=x1*y2-x2*y1;
    if(aria<0)aria=-aria;
    printf("%lf",aria/2.0);
    return 0;
}