n = int(input())
P = [list(map(int, input().split())) for _ in range(n)]

DP = [[0 for _ in range(2)] for _ in range(n + 1)]
for i in range(n):
    DP[i + 1][0] = max(DP[i + 1][0], DP[i][0] + P[i][0])
    if i >= 1:
        DP[i + 1][0] = max(DP[i + 1][0], DP[i][1] + 2 * P[i][0])
    DP[i + 1][1] = max(DP[i + 1][1], DP[i][0] + P[i][1])
    if i >= 1:
        DP[i + 1][1] = max(DP[i + 1][1], DP[i][1] + 2 * P[i][1])
print(max(DP[n]))