n = int(input())
d = [0] * 4
for _ in range(n):
    a, b = map(int, input().split())
    d[min((a & -a).bit_length() - 1, 3)] += b
ans = d[3]
t = min(d[1], d[2])
ans += t
d[1] -= t
d[2] -= t
ans += d[2] // 2 + d[1] // 3
print(ans)