#include <stdio.h>

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

/** Application main entry point. */
int main (
  int     argc,
  char  * argv[ ]
  )
{
  for ( ; ; )
  {
    double res = 0.;
    int x1, y1
      , x2, y2;
    int n;

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

    scanf ( "%d%d%d%d", &x1, &y1, &x2, &y2 );
    n -= 2;

    while ( n-- )
    {
      int x3, y3;
      int vx1, vy1
        , vx2, vy2;
      double vp;

      scanf ( "%d%d", &x3, &y3 );

      vx1 = x2 - x1;  vy1 = y2 - y1;
      vx2 = x3 - x1;  vy2 = y3 - y1;
      vp  = vx1 * vy2 - vy1 * vx2;

      res += DOUBLE( vp ) / 2.;

      x2 = x3;  y2 = y3;
    }

    printf ( "%.1f\n", ABS( res ) );
  }

  return ( 0 );
}