19554번 - Guess the number 출처다국어인터랙티브

Write a program that guesses an integer X in the range 1, . . . , N.

This is an interactive task. When your program starts, the first line of input contains the integer N (1 ≤ N ≤ 10⁹). The value of X is kept secret by the grading system.

Then your program can issue queries by writing them to the output in the form “? P”, where P is an integer (1 ≤ P ≤ N). The grading system writes the response in the next line of the input. The response is −1 if P < X, 0 if P = X, and 1 if P > X. Your program may issue at most 50 queries in each test case.

When your program wants to guess the number X, it should output “= X” (1 ≤ X ≤ N) and terminate. The grading system will not respond to this output and will not accept further queries after that.