612/C : Replace To Make Regular Bracket Sequence

#include <bits/stdc++.h>
using namespace std;

#define ll long long

stack<char> x;
string s;

int main(){

    ll ans = 0;

    cin >> s;
    if(s.size()%2 == 1){
        cout << "Impossible";
        return 0;
    }
    for(int i=0;i<s.size();i++){
        if(x.empty())
            x.push(s[i]);
        else if(x.top() == '('){
            switch(s[i]){
                case ')':
                    x.pop();
                    break;
                case '}':
                case ']':
                case '>':
                    ans++;
                    x.pop();
                    break;
                default:
                    x.push(s[i]);
                    break;
            }
        }
        else if(x.top() == '{'){
            switch(s[i]){
                case '}':
                    x.pop();
                    break;
                case ')':
                case ']':
                case '>':
                    ans++;
                    x.pop();
                    break;
                default:
                    x.push(s[i]);
                    break;
            }
        }
        else if(x.top() == '['){
            switch(s[i]){
                case ']':
                    x.pop();
                    break;
                case ')':
                case '}':
                case '>':
                    ans++;
                    x.pop();
                    break;
                default:
                    x.push(s[i]);
                    break;
            }
        }
        else if(x.top() == '<'){
            switch(s[i]){
                case '>':
                    x.pop();
                    break;
                case ')':
                case ']':
                case '}':
                    ans++;
                    x.pop();
                    break;
                default:
                    x.push(s[i]);
                    break;
            }
        }
        else{
            cout << "Impossible";
            return 0;
        }

    }
    if(x.size()==0)
        cout << ans;
    else
        cout << "Impossible";

    return 0;
}