#include <cstdio>

using namespace std;

struct pct
{
    long double x,y;
}Pol[100005];

//ifstream f("aria.in");
//ofstream e("aria.out");

int n;
long double A=0;

void citire()
{
    scanf("%d",&n);
    for(int i=0;i<n;i++)
    {
        scanf("%Lf %Lf",&Pol[i].x,&Pol[i].y);
    }
}


double Trunghi(pct A,pct B)
{
    return 0.5*((A.x*B.y)-(B.x*A.y));
}


void arie()
{
    long double A1;
    for(int i=0;i<n-1;i++)
        {
        A1=Trunghi(Pol[i],Pol[i+1]);
        A+=A1;
        }

    A1=Trunghi(Pol[n-1],Pol[0]);
    A+=A1;

    printf("%Lf",A);
}

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