#include<iostream>
#include<fstream>
#include<iomanip>
using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

#define ld double
#define MaxN 100100

int N;
ld Sol;
ld X[MaxN],Y[MaxN];

void citire(void)
{
    f >> N;
    for(int i=1;i<=N;i++)
        f >> X[i] >> Y[i];
}

void Rezolvare(void)
{
    X[N+1] = X[1];
    Y[N+1] = Y[1];

    for(int i=1;i<=N;i++)
        Sol += (ld)0.5 * (X[i]*Y[i+1] - Y[i]*X[i+1]);
}

int main()
{
    citire();
    Rezolvare();

    g << setprecision(32) << (Sol < 0 ? (ld)-1.0 * Sol : Sol);
}