#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int a, b, c, d, e = 0, f = 0, i, j;
  string s, x = "", y = "", z = "";
  cin >> s;
  c = -1;
  for (i = s.size() - 1; i >= 0; i--) {
    if (s[i] != 'a') {
      z += s[i];
      e++;
    } else {
      c = i;
      break;
    }
  }
  if (e != 0) {
    for (i = z.size() - 1; i >= 0; i--) {
      x += z[i];
    }
  }
  z = "";
  for (i = 0; i < c; i++) {
    if (s[i] != 'a') {
      y += s[i];
      f++;
    }
    z += s[i];
  }
  z += 'a';
  if (e < f) {
    cout << ":(" << endl;
    return 0;
  } else if (e == 0 && f == 0) {
    cout << z << endl;
    return 0;
  }
  if (c == -1) {
    if (s.size() % 2 == 0) {
      a = s.size() / 2;
      for (i = 0; i < s.size() / 2; i++) {
        if (x[i] != x[a]) {
          cout << ":(" << endl;
          return 0;
        }
        a++;
      }
      for (i = 0; i < s.size() / 2; i++) cout << x[i];
      cout << endl;
      return 0;
    } else {
      cout << ":(" << endl;
      return 0;
    }
  }
  c = x.size() + y.size();
  if (c % 2 != 0 || y.size() > x.size()) {
    cout << ":(" << endl;
    return 0;
  }
  a = y.size();
  b = x.size();
  c = 0;
  while (a < b) {
    a++;
    b--;
    y += x[c];
    c++;
  }
  for (i = 0; i < y.size(); i++) {
    if (y[i] != x[i + c]) {
      cout << ":(" << endl;
      return 0;
    }
  }
  for (i = 0; i < c; i++) {
    z += x[i];
  }
  cout << z << endl;
  return 0;
}
