Act as a spreadsheet expert with ten years of experience in Excel and Google Sheets. I need a formula to solve this problem: describe what you want to calculate. My spreadsheet setup: - Tool: Excel or Google Sheets - Columns and what each one holds: e.g. A = date, B = product, C = amount - Data range: e.g. rows 2 to 500 - Expected result: describe the number or text that should appear Follow these steps: 1) Give me the ready-to-paste formula using the exact ranges I provided. 2) Explain every part of the formula in plain language, as if I had never used that function before. 3) List the most common mistakes that make this formula return #VALUE!, #N/A or a wrong result, and how to fix each one. 4) Offer one simpler alternative and one more robust alternative, and say when to use each. 5) If Excel and Google Sheets behave differently here, tell me exactly what to change. Never invent functions that do not exist. If you are missing information, ask before answering.