#include <algorithm>
#include <iostream>
#include <stack>
#include <string>
#include <vector>

using namespace std;

vector<string> ans;
stack<string> st;

bool check(const string& cur, const string s) {
    if (cur.size() * 2 - 1 > s.size()) {
        return false;
    }
    vector<int> a(100, 0);
    for (size_t i = 0; i < cur.size(); i++) {
        for (size_t j = 0; j < cur.size(); j++) {
            a[i + j] += (cur[i] - '0') * (cur[j] - '0') % 10;
            a[i + j] %= 10;
        }
    }
    if (cur.size() * 2 - 1 == s.size()) {
        for (size_t i = 0; i < cur.size() * 2 - 1; i++) {
            if (s[i] - '0' != a[i]) {
                return false;
            }
        }
        return true;
    }
    for (size_t i = 0; i < cur.size(); i++) {
        if (s[i] - '0' != a[i]) {
            return false;
        }
    }
    return true;
}

int main() {
    string s; cin >> s;
    reverse(s.begin(), s.end());
    st.push("");
    while (!st.empty()) {
        string cur = st.top();
        st.pop();
        for (size_t i = 0; i < 10; i++) {
            cur.push_back(i + '0');
            if (check(cur, s)) {
                if (cur.size() * 2 - 1 == s.size()) {
                    ans.push_back(cur);
                } else {
                    st.push(cur);
                }
            }
            cur.pop_back();
        }
    }

    for (auto& item : ans) {
        reverse(item.begin(), item.end());
    }

    sort(ans.begin(), ans.end());
    if (ans.empty()) {
        cout << -1 << endl;
    } else {
        cout << ans[0] << endl;
    }
    /*
    cout << ans.size() << endl;
    for (auto& item : ans) {
        cout << item << endl;
    }
    */
    return 0;
}