## https://yukicoder.me/problems/no/1160


def main():
    N = int(input())
    pa = []
    for _ in range(N):
        p, a = map(int, input().split())
        pa.append((p, a))
    
    dp = [[-1, -1] for _ in range(N + 1)]
    dp[0] = [0, -1]
    for i in range(N):
        p, a = pa[i]

        # 前のターンでボーナスpin倒さない場合
        if dp[i][0] != -1:
            if p != a:
                dp[i + 1][0] = max(dp[i + 1][0], dp[i][0] + p)
                dp[i + 1][1] = max(dp[i + 1][1], dp[i][0] + a)
            else:
                dp[i + 1][1] = max(dp[i + 1][1], dp[i][0] + p)
        
        if dp[i][1] != -1:
            if p != a:
                dp[i + 1][0] = max(dp[i + 1][1], dp[i][1] + 2 * p)
                dp[i + 1][1] = max(dp[i + 1][1], dp[i][1] + 2 * a)
            else:
                dp[i + 1][1] = max(dp[i + 1][1], dp[i][1] + 2 * p)

    answer = max(dp[-1])
    print(answer)
        

if __name__ == "__main__":
    main()