#!/usr/bin/gawk -f

BEGIN{
  FS = OFS = " ";
  area = 0;
  first = 0;
  x = 0;
  y = 0;
  oracle=0;
}

first==0{
  first = 1;
  x = $1;
  y = $2;
  next;
}

first==1{
  l = $1 - x;
  rh = y;
  th = $2 - y;

  area += l * rh + 0.5 * l + th;

  x = $1;
  y = $2;
}

END{
  area += (100-x) * y;
  print area;
}