#include <bits/stdc++.h>
using namespace std;
int main() {
  long long x;
  cin >> x;
  string s;
  long long temp1 = sqrt(x);
  stringstream t1;
  t1 << temp1;
  t1 >> s;
  long long temp2 = 0;
  for (long long i = 0; i < s.size(); ++i) temp2 += (s[i] - 48);
  if (((temp2 * temp1) + (temp1 * temp1)) == x)
    cout << temp1 << endl;
  else {
    long long temp3 = temp1, counter = 0;
    while (counter <= 1000) {
      temp3--;
      counter++;
      long long temp4 = 0;
      string s2;
      stringstream t2;
      t2 << temp3;
      t2 >> s2;
      for (long long i = 0; i < s2.size(); ++i) temp4 += (s2[i] - 48);
      if (((temp3 * temp3) + (temp3 * temp4)) == x) {
        cout << temp3 << endl;
        break;
      } else if (temp3 == (int)(temp1 / 2)) {
        cout << -1 << endl;
        break;
      }
    }
    if (counter >= 1000) cout << -1 << endl;
  }
  return 0;
}
