import sys, math, itertools, functools, collections
input = sys.stdin.readline

def solve(testcase):
    m, r, n = map(int, input().split())
    x = [*map(int, input().split())]
    if x[0] > r or x[-1]+r < m or any(j-i > 2*r for i, j in zip(x, x[1:])):
        print('Case #'+str(testcase)+':', 'IMPOSSIBLE')
        return
    ans = 0
    illuminate = 0
    cur = 0
    while illuminate < m:
        while cur+1 < n and x[cur+1]-r <= illuminate:
            cur += 1
        illuminate = min(m, x[cur]+r)
        cur += 1
        ans += 1
    print('Case #'+str(testcase)+':', ans)


for testcase in range(1, int(input())+1):
    solve(testcase)