link。

代码：

#include <bits/stdc++.h>
#define int long long
#define IOS ios::sync_with_stdio(false), cin.tie(0)
#define rep(i, a, b) for (int i = (a); i <= (b); i++)
#define dep(i, a, b) for (int i = (a); i >= (b); i--)
using namespace std;
int T;
string s;
signed main()
{
    freopen("string.in", "r", stdin);
    freopen("string.out", "w", stdout);
    IOS;
    cin >> T;
    while (T--)
    {
        cin >> s;
        int len = s.size();
        string s1 = "", s2 = "", x = "";
        rep(i, 0, len - 1)
        {
            if (s[i] != 'a')
                x += s[i];
        }
        int lenx = x.size();
        bool flag = 0;
        rep(i, 0, lenx - 1)
        {
            s1 = x.substr(0, i + 1);
            s2 = x.substr(i + 1, lenx - i);
            // cout << s1 << " " << s2 << endl;
            if (s1 == s2)
            {
                flag = 1;
                break;
            }
        }
        string u = s.substr(len - s2.size(), s2.size());
        int lenu = u.size();
        // cout << u << endl;
        rep(i, 0, lenu)
        {
            if (u[i] == 'a')
            {
                flag = 0;
                break;
            }
        }
        if (flag == 0)
        {
            cout << ":(\n";
            continue;
        }
        rep(i, 0, len - lenu - 1)
            cout << s[i];
        cout << endl;
    }
    return 0;
}

自认为题目很简单（虽然死活都调不过），蒟蒻的思路也不难，所以就不过多解释了qwq。