RPN or Reverse Polish Notation is a notation for calculators to be less ambiguous. The last numbers use the operator to their right, repeat.

• 2 5 8 5 - × +
• (8 - 5 = 3)
• 2 5 3 × +
• (5 × 3 = 15)
• 15 2 +
• (15 + 2 = 17)
• 17