#include <stdio.h>

#define ABS(X) \
  ( (X) < 0 ? -(X) : (X) )

/** Application main entry point. */
int main (
  int     argc,
  char  * argv[ ]
  )
{
  for ( ; ; )
  {
    int res = 0.;
    int xi, yi
      , xp, yp;
    int n;

    scanf ( "%d", &n );
    if ( !( n ) ) break ;

    scanf ( "%d%d", &xi, &yi ), --n;
    xp = xi;  yp = yi;

    while ( n-- )
    {
      int xc, yc;

      scanf ( "%d%d", &xc, &yc );

      res += ( xp - xc ) * ( yp + yc );

      xp = xc;  yp = yc;
    }
    res += ( xp - xi ) * ( yp + yi );
    res  = ABS( res );

    printf ( "%d.%01d\n", res / 2, 5 * ( res % 2 ) );
  }

  return ( 0 );
}