How to sum a cell with another cells in different worksheets

Problem: To sum cells from another sheets in Excel

 Formulae for the example shown on picture is:
=SUM(A1;+February!A1;+March!A1)

to sum sheets named after months- in this example with cells to be summed are found at A1 address, and result to be displayed in first sheet (that's why it's called A1 - it doesn't have January in name)

If we want sum to be displayed in new sheet, the same formulae would be:
=SUM(January!A1;+February!A1;+March!A1)

or with cells we want to calculate are found at A1:
=SUM(name-of-the-first-sheet!A1+name-of-the-other-sheet!A1)