max = 0
r = 1000
n = int(input())
for i in range(n):
    ab = str(input())
    a = int(ab.split(" ")[0])
    b = int(ab.split(" ")[1])
    if a > b:
        max += a
        if a-b < r and (a-b) % 3 > 0:
            r = a - b
    else:
        max += b
        if b-a < r and(b-a) % 3 > 0:
            r = b - a
if max % 3 == 0:
    print(str(max-r))
else:
    print(str(max))