#include <iostream>
#include <cstdio>

using namespace std;




struct Puncte
{
    int x;
    int y;
}p[100001];


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


int Determinant(Puncte a, Puncte b)
{
    int Det;
    Det=(a.x * b.y)-((b.x)*(a.y));
}


void Rezolvare(int n)
{
    int aria;
    for (int i=0;i<n;i++)
    {
        aria+=(Determinant(p[i],p[i+1]))/2;
    }
    cout<<aria;
}


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