#include <bits/stdc++.h>
using namespace std;
template <class _T>
inline _T sqr(const _T& first) {
  return first * first;
}
template <class _T>
inline string tostr(const _T& a) {
  ostringstream os("");
  os << a;
  return os.str();
}
template <class _T>
inline istream& operator<<(istream& is, const _T& a) {
  is.putback(a);
  return is;
}
const long double PI = 3.1415926535897932384626433832795;
const long double EPS = 1e-11;
signed long long n;
signed long long s(signed long long first) {
  signed long long res = 0;
  while (first) {
    res += first % 10;
    first /= 10;
  }
  return res;
}
int main() {
  cout << setiosflags(ios::fixed) << setprecision(10);
  cin >> n;
  for (signed long long first =
           max((signed long long)1, (signed long long)sqrt(n) - 1000);
       first <= (signed long long)sqrt(n) + 1000; first++) {
    if (first * first + s(first) * first == n) {
      cout << first;
      return 0;
    }
  }
  cout << -1;
  return 0;
}
