N=int(input())
C=[0,0,0]
for i in range(N):
  a,b=map(int, input().split())
  if a%8==0:
    C[2]+=b 
  elif a%4==0:
    C[1]+=b 
  elif a%2==0:
    C[0]+=b 
    
ans=C[-1]
p=min(C[0],C[1]) 
ans+=p;C[0]-=p;C[1]-=p 
ans+=C[1]//2+C[0]//3
print(ans)