Sickly Yunho

Yunho is in poor health, so he takes medicine three times a day: morning, noon, and night. The medicine he picked up covers $N$ days, so the strip holds $3N$ doses.

Yunho is a perfectionist, so he never pulls a dose out of the middle of the row of $3N$ packets. Each time he takes one dose from the front end or the back end of the row. The dose he takes has to be the kind he is due to swallow. He starts with the morning dose of day 1, then noon, then night, then the morning dose of day 2, and so on. If neither end holds the dose he is due to take, Yunho stops there.

Find the largest number of doses Yunho can swallow.

The program reads from standard input.

The first line holds $N$, the number of days the medicine covers. ($1 \le N \le 500$)

The second line holds the kinds of all $3N$ doses in the order they lie on the strip, with no spaces. A morning dose is B, a noon dose is L, and a night dose is D. Each of the three letters appears exactly $N$ times.

The program writes to standard output.

Print the largest number of doses Yunho can swallow on one line.