#include<cstdio>
using namespace std;
int i,n;
double d,s,x[100009],y[100009];
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&n);
    for(i=1;i<=n;i++)
    {
        scanf("%lf%lf",&x[i],&y[i]);
        if(i>1) d=(double)x[i-1]*y[i]-x[i]*y[i-1];
        s=s+d;
    }
    s=(double)s+x[n]*y[1]-y[n]*x[1];
    s=(double)s/2;
    printf("%lf",s);
    return 0;
}