#include<cstdio>
#define vreau_sa_iau_100 printf("%.5lf",a/2.0);
#define citire_ultra_secreta scanf("%lf%lf",&x[i],&y[i]);
#define nu_vreau_sa_scriu_for for(i=1;i<=n;++i)
#define formula_jmenoasa a+=x[i]*y[i+1]-x[i+1]*y[i];
int i,n;
double a,x[100007],y[100007];
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&n);
    nu_vreau_sa_scriu_for
        citire_ultra_secreta
    x[n+1]=x[1];
    y[n+1]=y[1];
    nu_vreau_sa_scriu_for
        formula_jmenoasa
    vreau_sa_iau_100
    return 0;
}