#include <cstdio>
#include <cmath>

using namespace std;

#define NMAX 100015

double X[NMAX],Y[NMAX], S;
int N;

void Read()
{

    freopen("aria.in","r",stdin);

    scanf("%d",&N);

    for(int i=1;i<=N;++i)
        scanf("%lf %lf",&X[i],&Y[i]);
}

void TSFH()
{
    X[N+1] = X[1]; Y[N+1] = Y[1];
    for(int i=1;i<=N;i++)
    {
        S += X[i] * Y[i+1] - X[i+1] * Y[i];
    }
}

void Print()
{
    freopen("aria.out","w",stdout);
    printf ("%lf", fabs (S / 2.0));
}

int main()
{
    Read();
    TSFH();
    Print();
    return 0;
}