using System;

using System.Collections.Generic;

​

public class Program

{

    static Dictionary<string, string> cells = new Dictionary<string, string>();

    static Dictionary<string, int> cellValues = new Dictionary<string, int>();

    static HashSet<string> visitedCells = new HashSet<string>();

    static void SetCell(string cellAddr, string cellValue)

    {

        if (cellValue == null || cellValue.Length == 0) cells.Remove(cellAddr);

        else cells[cellAddr] = cellValue;

        cellValues.Clear();

        visitedCells.Clear();

    }

​

    // 0 on empty cell, exception on cycle or invalid cell

    static int GetValue(string cellAddr)

    {

        if (cellValues.ContainsKey(cellAddr)) return cellValues[cellAddr];

        if (visitedCells.Contains(cellAddr)) throw new Exception("cells form cycle");

        visitedCells.Add(cellAddr);

        string cell = "";

        int cellValue = 0;

        if (!cells.TryGetValue(cellAddr, out cell)) cellValue = 0;

        else

        {

            char firstChar = cell[0];

            if (firstChar == '=') // formula

            {

                // for real product would do more input validation so unsupported characters cause error

                var sumCells = cell.Substring(1).Split('+');

                foreach (var sumCell in sumCells) cellValue += GetValue(sumCell);

            }

            else if (!int.TryParse(cell, out cellValue)) throw new Exception("Invalid cell");

        }

        cellValues[cellAddr] = cellValue;

        return cellValue;

    }

​

    public static void Main()

    {

        SetCell("B2", "98");

        SetCell("F18", "1");

        SetCell("F22", "=F18+F18");

        SetCell("B52", "=B2+F22+B2+F22+F35");

        Console.WriteLine("Cell B52: " + GetValue("B52"));

        SetCell("F18", "101");

        Console.WriteLine("Cell B52: " + GetValue("B52"));

    }

}