t = int(input().strip())

for _ in range(t):
    n, m = [int(x) for x in input().strip().split()]
    failed = False
    answer = n-1
    for __ in range(m):
        a, b = [int(x) for x in input().strip().split()]
        if a>b:
            failed = True
        elif a+1 == b:
            answer -= 1
    print( -1 if failed else answer )