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

print(dp[N][0])