#include <iostream>
#include <fstream>
#include <math.h>
#define NMAX 100000

using namespace std;
float x[NMAX],y[NMAX];
int N;
float S;
ifstream fin("aria.in");
ofstream fout("aria.out");
void read()
{
    int i;
    fin>>N;
    for(i=1;i<=N;i++)
        fin>>x[i]>>y[i];
}

int main()
{
    int i;
    read();
    x[N+1]=x[1];
    y[N+1]=y[1];
    for(i=1;i<=N;i++)
    {
        int aux=x[i]*y[i+1]-x[i+1]*y[i];
        S+=aux;
    }

    fout<<S/2;
    return 0;
}