# UUID: 71b8761b-e1db-448f-9a74-45da67cdd91f
from sys import stdin, setrecursionlimit
input = stdin.readline
setrecursionlimit(10**6)

N = int(input())
negy=[]
ot=[]
for i in range(1,N+1):
    S, M = map(int,input().split())
    if M==4: 
        negy.append((S,i))
    elif M>4:
        ot.append((S,i))

negy.sort()
ot.sort()

def test(k):
    ans = []
    i = len(negy)-1
    j = len(ot)-1
    for day in range(k,0,-1):
        if 0 <= i and negy[i][0] >= day:
            if 0 <= j and ot[j][0] >= day:
                ans.append((negy[i][1],ot[j][1]))
                i -= 1
                j -= 1
            else:
                return False, []
        elif 1 <= j and ot[j-1][0] >= day:
            ans.append((ot[j][1],ot[j-1][1]))
            j -= 2
        else:
            return False, []
    return True, ans

lo = 0
hi = 2001
res = []
while hi-lo > 1:
    mid = (hi+lo)//2
    ok, ans = test(mid)
    if ok:
        lo = mid
        res = ans
    else:
        hi = mid
print(lo)
res.reverse()
for line in res:
    print(*line)