t = str(input())
s = t.replace('a','')
s_ = s[int(len(s)/2):]
s = s[:int(len(s)/2)]
if(s==s_):
s = t[0:len(t)-len(s_)]
if t == s+s_:
print(s)
else:
print(":(")
else:
print(":(")
#include<iostream>
#include<cstring>
using namespace std;
int main()
{
int num=0,fl=0,fls=0;
bool flag = 1;
string str;
cin >> str;
for (int i = 0; i < str.size(); i++)
{
if (str[i] == 'a')
{
num++;
fl = i;
}
}
fls = (str.size() + num) / 2;
if (fls < fl)flag == 0;
if (str.size() - num & 1)flag = 0;
int l = 0, r = fls;
//cout << "ll rr" << l << "///" << r << endl;
while (r <str.size())
{
if (str[l] == 'a')
{
l++;
continue;
}
if (str[l] != str[r])
{
flag = 0;
//cout << "l=" << l<<"r="<<r<<"str[l]="<<str[l]<<"str[r]="<<str[r]<<endl;
break;
}
l++;
r++;
}
if (flag)
{
for (int i = 0; i < fls; i++)
cout << str[i];
}
else cout << ":(" << endl;
}
343B - Alternating Current | 758B - Blown Garland |
1681B - Card Trick | 1592A - Gamer Hemose |
493D - Vasya and Chess | 1485A - Add and Divide |
337B - Routine Problem | 1392D - Omkar and Bed Wars |
76E - Points | 762C - Two strings |
802M - April Fools' Problem (easy) | 577B - Modulo Sum |
1555B - Two Tables | 1686A - Everything Everywhere All But One |
1469B - Red and Blue | 1257B - Magic Stick |
18C - Stripe | 1203B - Equal Rectangles |
1536A - Omkar and Bad Story | 1509A - Average Height |
1506C - Double-ended Strings | 340A - The Wall |
377A - Maze | 500A - New Year Transportation |
908D - New Year and Arbitrary Arrangement | 199A - Hexadecimal's theorem |
519C - A and B and Team Training | 631A - Interview |
961B - Lecture Sleep | 522A - Reposts |