Binary search-778A CODEFORCES

https://codeforces.com/contest/778/problem/A

can anyone explain to me how we are using binary search in this question?
basically, what I understand is that we are given a string “s” and a string “t” we want the string “t” to be a subsequence of string “s” after deleting nth letter from it what I don’t understand how we are using binary search property in this?