#include <cstdio>
using namespace std;
int n;
double aria,xi,yi,x_old,y_old,x_new,y_new;
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&n);
    scanf("%lf%lf",&xi,&yi);
    x_old=xi;y_old=yi;
    for(n--;n;n--)
    {
        scanf("%lf%lf",&x_new,&y_new);
        aria+=(y_new+y_old)*(x_new-x_old)/2;
        x_old=x_new;y_old=y_new;
    }
    x_new=xi;y_new=yi;
    aria+=(y_new+y_old)*(x_new-x_old)/2;
    if(aria<0)aria=-aria;
    printf("%lf",aria);
    return 0;
}