#include <bits/stdc++.h>
using namespace std;
double PI = 3.1415926536;
int dX[] = {1, -1, 0, 0};
int dY[] = {0, 0, 1, -1};
int Sum(int x) {
  string s;
  int ret = 0;
  stringstream ss;
  ss << x;
  ss >> s;
  ss.clear();
  for (int i = (0); i < ((int)s.size()); i++) ret += s[i] - 48;
  return ret;
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long int n, res = -1;
  cin >> n;
  for (int i = (1); i < (9 * 18 + 1); i++) {
    long long int disc = i * i + 4 * n, root = sqrt(disc);
    if (root * root == disc) {
      long long int t = -i + root;
      if (t % 2 == 0 && Sum(t / 2) == i) {
        if (res == -1)
          res = t / 2;
        else
          res = min(res, t / 2);
      }
    }
  }
  cout << res;
}
