text: #include <bits/stdc++.h>
  
  using namespace std;
  
  using ll = long long;
  using P = pair<ll, ll>;
  using Graph = vector<vector<ll>>;
  #define rep(i, n) for(ll i=0;i<(ll)(n);i++)
  #define rep2(i, m, n) for(ll i=m;i<(ll)(n);i++)
  #define rrep(i, n, m) for(ll i=n;i>=(ll)(m);i--)
  const int dx[4] = {1, 0, -1, 0};
  const int dy[4] = {0, 1, 0, -1};
  const int ddx[8] = {0, 1, 1, 1, 0, -1, -1, -1};
  const int ddy[8] = {1, 1, 0, -1, -1, -1, 0, 1};
  const ll MOD = 1000000007;
  const ll INF = 1000000000000000000L;
  #ifdef __DEBUG
  
  /**
   * For DEBUG
   * https://github.com/ta7uw/cpp-pyprint
   */
  #include "cpp-pyprint/pyprint.h"
  
  #endif
  
  void Main() {
      int N;
      cin >> N;
      rep(i, N + 1) {
          if (i * 108 / 100 == N) {
              cout << i << '\n';
              return;
          }
      }
      cout << ":(" << '\n';
  }
  
  int main() {
      cin.tie(nullptr);
      ios::sync_with_stdio(false);
      cout << fixed << setprecision(15);
      Main();
      return 0;
  }
  