import sys
from collections import deque, defaultdict
import heapq
import copy

######################################################################

#UnionFind
class UnionFind():
    def __init__(self, n):
        self.n = n
        self.parents = [-1] * n
        self.group = n
    def find(self, x):
        if self.parents[x] < 0:
            return x
        else:
            self.parents[x] = self.find(self.parents[x])
            return self.parents[x]
    def union(self, x, y):
        x = self.find(x)
        y = self.find(y)
        if x == y:
            return
        if self.parents[x] > self.parents[y]:
            x, y = y, x
        self.parents[x] += self.parents[y]
        self.parents[y] = x
        self.group -= 1
    def size(self, x):
        return -self.parents[self.find(x)]
    def same(self, x, y):
        return self.find(x) == self.find(y)
    def members(self, x):
        root = self.find(x)
        return [i for i in range(self.n) if self.find(i) == root]
    def roots(self):
        return [i for i, x in enumerate(self.parents) if x < 0]
    def group_count(self):
        return self.group
    def all_group_members(self):
        return {r: self.members(r) for r in self.roots()}
    def __str__(self):
        return ' '.join('{}: {}'.format(r, self.members(r)) for r in self.roots())

######################################################################

DEBUG = 0
if __name__ != "__main__":
    DEBUG = 1
    import DebugLib as dl

def MyInput():
    global DEBUG
    if DEBUG: return dl.DebugInput()
    else: return input()


def MyPrint(*arg, **keys):
    global DEBUG
    if DEBUG: return dl.DebugPrint(*arg, **keys)
    else: return print(*arg, **keys)

######################################################################

def InputInfo():
    ar = list(map(int,MyInput().split()))
    n = ar[0]
    s = ar[1:]
    return n,s

def OutputAns():
    MyPrint(len(outInfo))
    for ar in outInfo:
        MyPrint(*ar)
    if not DEBUG:
        sys.stdout.flush()

N = 5400
T, R = map(int,MyInput().split())

uf = UnionFind(N+1)

q = []
idx = 1
nowidx = 1
for t in range(T):
    n, s = InputInfo()
    outInfo = []
    for x in s:
        if idx == 1:
            idx += 1
            continue
        
        while R <= uf.size(nowidx):
            nowidx += 1
        
        uf.union(idx, nowidx)
        outInfo.append([idx, nowidx])
        
        idx += 1
    
    OutputAns()