import cp.
main =>
  N = read_int(),
  maxX = 0,
  maxY = 0,
  x = 0,
  y = 0,
  F = 0,
  while (F < N)
    x: = read_int(),
    if(x > maxX) then
      maxX := x
    end,
    y: = read_int(),
    if(y > maxY) then
      maxY := y
    end,
    F:=F+1
  end,
  println(((maxX*maxY)/2)).