CodeForces 612C Replace To Make Regular Bracket Sequence

C. Replace To Make Regular Bracket Sequence

time limit per test

1 second

memory limit per test

256 megabytes

input

standard input

output

standard output

You are given string s consists of opening and closing brackets of four kinds <>, {}, [], (). There are two types of brackets: opening and closing. You can replace any bracket by another of the same type. For example, you can replace < by the bracket {, but you can't replace it by ) or >.

The following definition of a regular bracket sequence is well-known, so you can be familiar with it.

Let's define a regular bracket sequence (RBS). Empty string is RBS. Let s1 and s2 be a RBS then the strings <s1>s2, {s1}s2, [s1]s2,(s1)s2 are also RBS.

For example the string "[[(){}]<>]" is RBS, but the strings "[)()" and "][()()" are not.

Determine the least number of replaces to make the string s RBS.

Input

The only line contains a non empty string s, consisting of only opening and closing brackets of four kinds. The length of s does not exceed 106.

Output

If it's impossible to get RBS from s print Impossible.

Otherwise print the least number of replaces needed to get RBS from s.

Examples

input

[<}){}

output

2

input

{()}[]

output

0

input

]]

output

Impossible

一个括号匹配的题。不算是第一次做，不过第一次看到这个题的时候迷迷糊糊的看了题解也感觉没懂，所以没往上贴。现在算是彻底明白了，来存个档。

题解：本题用到了栈。如果遇上左括号，就加进栈里。如果遇上右括号，就判断栈里的左括号是否和它匹配，不匹配就加一。不论匹不匹配，判断后都要让左括号出栈。

如果最后栈不为空，或者栈在循环结束前就为空，那么不论怎么改变，左右括号都不可能刚好匹配。

#include<stdio.h>#include<string.h>#include<algorithm>#include<stack>using namespace std;char ch[1000000+10];stack<int> s;int main(){int i;while(~scanf("%s",ch)){int l=strlen(ch);int flag=1;int ans=0;while(!s.empty()) s.pop();for(i=0;i<l;i++){if(ch[i]=='['||ch[i]=='<'||ch[i]=='('||ch[i]=='{')s.push(ch[i]);if(s.empty()){flag=0;break;}if(ch[i]==']'){if(s.top()!='[')ans++;s.pop();}else if(ch[i]=='}'){if(s.top()!='{')ans++;s.pop();}else if(ch[i]=='>'){if(s.top()!='<')ans++;s.pop();}else if(ch[i]==')'){if(s.top()!='(')ans++;s.pop();}}if(!s.empty()||!flag)printf("Impossible\n");elseprintf("%d\n",ans);}return 0;}