t = int(input())
for _ in range(t):
    n, m = map(int, input().split())
    ans = n-1
    for i in range(m):
        a, b = map(int, input().split())
        if a > b: ans = -1
        elif a + 1 == b: ans -= 1
    if ans < 0: print(-1)
    else: print(ans)