N = int(input())
P = [0 for _ in range(N)]
A = [0 for _ in range(N)]
DP = [[0, 0] for _ in range(N+1)]

for i in range(N):
    P[i], A[i] = map(int, input().split())
    if i != 0:
        DP[i+1][0] =  max(DP[i][0] + P[i], DP[i][1] + 2*P[i] )
        DP[i+1][1] =  max(DP[i][0] + A[i], DP[i][1] + 2*A[i] )
    else:
        DP[i+1][0] =  P[i] 
        DP[i+1][1] =  A[i]        
        
ANS = max(DP[N][0], DP[N][1])
print(ANS)