#include <bits/stdc++.h>
using namespace std;
long long int sod(long long int x) {
  long long int temp = x;
  long long int sum = 0;
  while (temp != 0) {
    sum += temp % 10;
    temp /= 10;
  }
  return sum;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  ;
  long long n, b;
  cin >> n;
  int maxsx = 9 * 20;
  long long ans = -1;
  for (int b = 0; b < maxsx; b++) {
    long long det = b * b + 4 * n;
    long long sq = sqrt(det);
    if (sq * sq == det) {
      long long x = -b + sq;
      long long y = -b - sq;
      if (x & 1) {
      } else {
        x /= 2;
        if (x > 0 && sod(x) == b) {
          if (ans == -1) {
            ans = x;
          } else {
            ans = min(ans, x);
          }
        }
      }
      if (y & 1) {
      } else {
        y /= 2;
        if (y > 0 && sod(y) == b) {
          if (ans == -1) {
            ans = y;
          } else {
            ans = min(ans, y);
          }
        }
      }
    }
  }
  cout << ans;
  return 0;
}
