Codeforces#420 Okabe and Boxes

题目网址：http://codeforces.com/contest/821/problem/C

题目：

C. Okabe and Boxes

time limit per test

3 seconds

memory limit per test

256 megabytes

input

standard input

output

standard output

Okabe and Super Hacker Daru are stacking and removing boxes. There are n boxes numbered from 1 to n. Initially there are no boxes on the stack.

Okabe, being a control freak, gives Daru 2n commands: n of which are to add a box to the top of the stack, and n of which are to remove a box from the top of the stack and throw it in the trash. Okabe wants Daru to throw away the boxes in the order from 1 to n. Of course, this means that it might be impossible for Daru to perform some of Okabe's remove commands, because the required box is not on the top of the stack.

That's why Daru can decide to wait until Okabe looks away and then reorder the boxes in the stack in any way he wants. He can do it at any point of time between Okabe's commands, but he can't add or remove boxes while he does it.

Tell Daru the minimum number of times he needs to reorder the boxes so that he can successfully complete all of Okabe's commands. It is guaranteed that every box is added before it is required to be removed.

Input

The first line of input contains the integer n (1 ≤ n ≤ 3·105) — the number of boxes.

Each of the next 2n lines of input starts with a string "add" or "remove". If the line starts with the "add", an integer x (1 ≤ x ≤ n) follows, indicating that Daru should add the box with number x to the top of the stack. 

It is guaranteed that exactly n lines contain "add" operations, all the boxes added are distinct, and n lines contain "remove" operations. It is also guaranteed that a box is always added before it is required to be removed.

Output

Print the minimum number of times Daru needs to reorder the boxes to successfully complete all of Okabe's commands.

Examples

input

3
add 1
remove
add 2
add 3
remove
remove

output

1

input

7
add 3
add 2
add 1
remove
add 4
remove
remove
remove
add 6
add 7
add 5
remove
remove
remove

output

2

Note

In the first sample, Daru should reorder the boxes after adding box 3 to the stack.

In the second sample, Daru should reorder the boxes after adding box 4 and box 7 to the stack.

思路：这道题算是技巧题，虽然题目跟栈联系起来，但其实只用到了栈先进后出的特点。刚开头用了很传统的思想，每次remove之前都检查一下，是否要排序，需要的话就排序一次，不出意外地超时了……后面看了大佬的代码，发现这道题的关键点在于，第i次remove时，栈内一定有i元素。所以我们只需要检查remove时，栈顶元素是否符合要求，如果不符合就直接清空栈，排序次数加一，如果符合则栈顶元素出栈。

代码：

#include 
#include 
#include 
#include 
using namespace std;
stacks;
int main(){
    int n;
    int cur=1;
    int res=0;
    scanf("%d",&n);
    for (int i=0; i<2*n; i++) {
        char str[10];
        int num;
        scanf("%s",str);
        if(strcmp(str, "add")==0){
            scanf("%d",&num);
            s.push(num);
        }else{
            if(!s.empty() && s.top()==cur){
                s.pop();
            }
            else{
                if(!s.empty())  res++;
                while (!s.empty()) {s.pop();}
            }
            cur++;
        }
    }
    printf("%d\n",res);
    return 0;
}