Just get the value of a cell

Workbook workbook = new Workbook();
Worksheet worksheet = workbook.Worksheets.Add();

worksheet.Cells[0, 0].SetValue("=SUM(A2, 3)"); // cell A1, equal to cell A2+3 or 13
worksheet.Cells[1, 0].SetValue(10); // cell A2

var rawValueA1 = worksheet.Cells[0, 0].GetValue().Value.RawValue; // returns the string "=SUM(A2, 3)"
var rawValueA2 = worksheet.Cells[1, 0].GetValue().Value.RawValue; // returns the string "10"

ICellValue cellValue = worksheet.Cells[0, 0].GetValue().Value; //This interface is implemented by the five supported Cell Value types. 


CellValueFormat format = worksheet.Cells[0, 0].GetFormat().Value; // A specific data format is required, in order to evaluate the resultValue of a formula. In this case we can use the one present in the cell itself.

string valueAsString = cellValue.GetValueAsString(format);   // returns the string "=SUM(A2, 3)" 
string resultValue = cellValue.GetResultValueAsString(format); // returns the string "13"

ICellValue cellValue = worksheet.Cells[0, 0].GetValue().Value; 

 FormulaCellValue formulaValue = cellValue as FormulaCellValue; //This cast allows the use of the GetResultValueAsCellValue() method
 if (formulaValue != null)
 {
          NumberCellValue numberCellValue = formulaValue.GetResultValueAsCellValue() as NumberCellValue; // This cast allows access to the Value property of the specific CellValue. Please make sure to check the appropriate CellValueType needed before casting.
          double result = numberCellValue.Value; // returns double 13
 }

You can read more about the supported Cell Values and their properties here - SpreadProcessing - Cell Value Types.

I hope this helps. Should you have any other questions do not hesitate to ask. Have a great day!