4. [6 points) We want to convert one string to another. It costs $10 to delete a letter, and $15 to replace a letter-importantly, note that it is not allowed to insert a new letter. We want to find the minimum cost to do this. For example, a string "HELLO WORLD can be converted to "YELLOWORC for $50: replace "H with Y, delete the space, replace L with C, and then delete the D. Suppose X X122 ... Xn is the old string, and we want to convert it to Y = Y1Y2 ... ym. Note that 0