Sry, I didn’t understand this: Sequence s[1], s[2], …, s[k] is called a subsequence of sequence a[1], a[2], …, a[n], if there will be such increasing sequence of indices i[1], i[2], …, i[k] (1 ≤ i[1] < i[2] < … < i[k] ≤ n), that a[i[j]] = s[j]. In other words, sequence s can be obtained from the a by deleting some elements.

that’s mean that if we have the sequence like 1 2 3 4 5 6, so sequence like 1 2 4 6 is subsequence or not? Pls tell me cauze this is the main thing in that problem.

Sry, for my english.