#include <iostream>
#include <fstream>

using namespace std;

int N, i;

double Rez = 0;

int main()
{

    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);

    scanf("%d", &N);

    long long X, Y, fX, fY;
    long long X2, Y2;
    for(i=1;i<=N+1;++i)
    {
        if(i == 1)
        {
            cin>>X>>Y;
            fX = X;
            fY = Y;
        }
        else
        {

            if(i == N + 1)
                X2 = fX, Y2 = fY;
            else
                cin>>X2>>Y2;

            //cout<<X<<" "<<Y<<" = "<<X2<<" "<<Y2<<endl;

            Rez += (X * Y2 - X2 * Y);

            X = X2;
            Y = Y2;
        }

    }
        printf("%.0f\n", Rez / 2);

    return 0;
}