#include <cstdio>

using namespace std;

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

int n;
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 trap(pct A,pct B)
{
   return ((A.y+B.y)*(B.x-A.x))/2;
}

void arie()
{
    for(int i=0;i<n-1;i++)
        A+=trap(Pol[i],Pol[i+1]);
    A+=trap(Pol[0],Pol[n-1]);
    A=(A>0)?A:-A;

    printf("%.5f",A);
}

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