import sys

input_data = sys.stdin.read().split()

n = int(input_data[0])
m = int(input_data[1])

graph = [[] for _ in range(n + 1)]
ptr = 2

for i in range(1, n + 1):
    while True:
        val = int(input_data[ptr])
        ptr += 1
        if val == 0:
            break
        graph[i].append(val)

matching = [0] * (m + 1)

def dfs(v):
    if used[v]:
        return False
    used[v] = True
    for to in graph[v]:
        if matching[to] == 0 or dfs(matching[to]):
            matching[to] = v
            return True
    return False

for i in range(1, n + 1):
    used = [False] * (n + 1)
    dfs(i)

result = []
for to in range(1, m + 1):
    if matching[to] != 0:
        result.append((matching[to], to))

print(len(result))
for u, v in result:
    print(u, v)