#include <bits/stdc++.h>

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

const int NMAX = 100005;
int x[NMAX],y[NMAX];

long long arie_triunghi(int x1,int y1,int x2,int y2,int x3,int y3)
{
    return abs(1LL*(x2-x1)*(y3-y1) - 1LL*(y2-y1)*(x3-x1)) / 2;
}

int main()
{
    int n;
    fin >> n;
    for(int i=1;i<=n;i++)
    {
        fin >> x[i] >> y[i];
    }
    long long arie=0;
    for(int i=2;i<=n-1;i++)
    {
        arie+=arie_triunghi(x[1],y[1],x[i],y[i],x[i+1],y[i+1]);
    }
    fout << arie;
    return 0;
}