N = int(input())
D = []
for i in range(1, N + 1):
    D.append(list(map(int, input().split())))
inf = 10 ** 18
dp = [[-inf, -inf] for i in range(N + 1)]
dp[0][0] = 0
for i in range(N):
    p, a = D[i]
    dp[i + 1][0] = max(dp[i][0] + p, dp[i][1] + 2 * p)
    dp[i + 1][1] = max(dp[i][0] + a, dp[i][1] + 2 * a)

print(max(dp[N]))