If the given words are not synonymous, output  'NO' in a single line. 

Otherwise, output 'YES' in the first line. The second line contains an integer $K$ ($K \le 16 \cdot N$) -- a number of synonymization required for the conversion. The following $K$ lines contain pairs of numbers that describe conversion of the first word into the second one. The first integer specifies a number of the first character participating in synonymization, and the second integer (always greater than the first one) -- a number of the last character.