N = int(input())
cnt = [0] * 4
for _ in range(N):
    a, b = map(int, input().split())
    two = 0
    while a % 2 == 0:
        a //= 2
        two += 1
    cnt[min(3, two)] += b
ans = cnt[3] + min(cnt[1], cnt[2]) + (cnt[1] - min(cnt[1], cnt[2])) // 3
print(ans)