#include <iostream>
#include <cstdio>

using namespace std;

int n , s , x[1000007] , y[1000007] ;

int main()
{
    freopen ("aria.in" , "r" , stdin ) ;
    freopen ("aria.out" , "w" , stdout ) ;
    scanf ("%d" , &n ) ;
    for ( int i = 1 ; i <= n ; ++i )
        scanf  ("%d%d" , &x[i] , &y[i] ) ;
    for ( int i = 1 ; i <= n ; ++i )
        {
            s+=x[i]*y[i+1]-x[i+1]*y[i];
            x[n+1]=x[1];
            y[n+1]=y[1];
        }
    s=s/2;
    printf ("%d" , s ) ;
    return 0;
}