n=int(input())
c=[0]*4
for i in range(n):
  a,b=map(int,input().split())
  for j in reversed(range(4)):
    if a%(2**j)==0:
      c[j]+=b
      break
a=0
a+=c[3]
g=min(c[2],c[1])
a+=g
c[2]-=g
c[1]-=g
a+=c[2]//2
a+=c[1]//3
print(a)