#include <iostream>

using namespace std;

int main()
{

    int n;
    int *x, *y,A=0;
    cin >>n;
    x = new int[n];
    y = new int[n];
    for (int i = 0; i<n; i++)
    {
        cin >> x[i] >> y[i];
    }
    for(int i = 0; i<n-1; i++)
        A = A + (1/2)*(x[i]*y[i+1] - x[i+1]*y[i]);

    cout<<A;
    return 0;
}