#include <cstdio>
#define nmax 100005

using namespace std;

double x[nmax],y[nmax];
int n;

void citire()
{
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
        scanf("%lf%lf",&x[i],&y[i]);
}

void rezolv()
{
    double s=0;
    x[n+1]=x[1];
    y[n+1]=y[1];
    for(int i=1;i<=n+1;i++)
        s+=x[i]*y[i+1]-x[i+1]*y[i];
    s/=2;
    if(s<0)
        s=-s;
    printf("%.5lf",s);
}

int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    citire();
    rezolv();
    return 0;
}