B"There is a square matrix, consisting of n rows and n columns of cells, both numbered from 1 to n . The cells are colored white or black. Cells from 1 to a_i are black, and cells from a_i+1 to n are white, in the i -th column. You want to place m integers in the matrix, from 1 to m . There are two rules: The beauty of the matrix is the number of such j that j+1 is written in the same row, in the next column as j (in the neighbouring cell to the right). What's the maximum possible beauty of the matrix? The first line contains a single integer t ( 1 <= t <= 10^4 ) -- the number of testcases. The first line of each testcase contains a single integer n ( 1 <= n <= 2 cdot 10^5 ) -- the size of the matrix. The second line contains n integers a_1, a_2, ... , a_n ( 0 <= a_i <= n ) -- the number of black cells in each column. The third line contains a single integer m ( 0 <= m <= sum limits_{i=1}^n n - a_i ) -- the number of integers you have to write in the matrix. Note that this number might not fit into a 32-bit integer data type. The sum of n over all testcases doesn't exceed 2 cdot 10^5 . For each testcase, print a single integer -- the maximum beauty of the matrix after you write all m integers in it. Note that there are no more integers than the white cells, so the answer always exists. "...