#include <bits/stdc++.h>

using namespace std;

string s;

map<string, int> ans;
int conv[30];

void dfs(string t) {
  if (t.size() == (s.size() + 1) / 2) {
    for (int i = 0; i < s.size(); i++) {
      if (s[i] - '0' != conv[i]) return;
    }
    reverse(t.begin(), t.end());
    ans[t];
    return;
  }
  for (int d = 0; d <= 9; d++) {
    if (!t.size()) {
      if (d * d % 10 == s[0] - '0') {
        conv[0] = d * d % 10;
        t += '0' + d;
        dfs(t);
        t.pop_back();
        conv[0] = 0;
      }
    }
    else {
      int ck = (conv[t.size()] + 2 * d * (t[0] - '0')) % 10;
      if (ck == s[t.size()] - '0') {
        for (int i = 0; i < t.size(); i++) (conv[t.size() + i] += 2 * d * (t[i] - '0')) %= 10;
        conv[2 * t.size()] = d * d % 10;
        t += '0' + d;
        dfs(t);
        t.pop_back();
        for (int i = 0; i < t.size(); i++) (conv[t.size() + i] -= 2 * d * (t[i] - '0')) %= 10, (conv[t.size() + i] += 10) %= 10;
        conv[2 * t.size()] = 0;
      }
    }
  }
} 

int main() {
  cin.tie(0)->sync_with_stdio(0);
  cin >> s;
  reverse(s.begin(), s.end());
  if (s.size() % 2 == 0) cout << -1 << '\n';
  else {
    dfs("");
    cout << (ans.size() ? (*ans.begin()).first : "-1") << '\n';
  }
  return 0;
}