Note that a wff is either a single sentence symbol ($1$ symbol), a negation of another wff ($n+3$ symbols), or a combination of two wffs using one of the four binary connective symbols ($n+m+3$ symbols), where $n$ and $m$ are the numbers of symbols in the wffs being used. Therefore, a wff can have $1,1+3=4,4+3=7$ symbols as well as $1+1+3=5,1+4+3=8,1+5+3=9$ symbols with further possibility to produce $k+3l$ symbols where $k\in\{7,8,9\}$ and $l\in\mathbb{N}$ . $2$ and $3$ symbols are impossible, since both $n+3$ and $n+m+3$ have to be greater than $3$ . $6$ is not possible as otherwise $6=n+3$ , implying $n=3$ , or $6=n+m+3$ , implying $n+m=3$ , implying $n$ or $m$ equal to $2$ . (Real life formulas may have any number of symbols, as the outermost parentheses are often omitted, and, besides, for example, parentheses are not used in a repetition of the negation operation, such as $\neg\neg A$ .)