Write a simple spreadsheet calculator. A spreadsheet is a table of cells arranged in rows and columns.
Every cell of the table holds either a number or a formula. A formula starts with the character = followed by one or more cell addresses. Consecutive addresses are separated by the character +. A formula contains no spaces at all. The value of a cell that holds a formula is the sum of the values of the cells listed in that formula.
A cell address is a column name followed immediately by a row number, with no space between them. The column names are A, B, C, ..., Z, AA, AB, AC, ..., AZ, BA, BB, BC, ..., BZ, CA, CB, CC, ..., CV, and in that order they stand for columns 1 through 100. Rows are numbered 1 through 100.