#include <iostream>
#include <fstream>

using namespace std;

int n,x1,y1,x2,y2,a=0;

void fct()
{
    ifstream fin("aria.in");
    fin>>n;
    fin>>x1>>y1;
    for(int i=1;i<n;i++)
    {
        fin>>x2>>y2;
        a+=(y2+y1)*(x2-x1)/2;
        x1=x2;
        y1=y2;
    }
    if(a<0)
        a=-a;
    ofstream fout("aria.out");
    fout<<a;
}

int main()
{
    fct();
    return 0;
}