#include <iostream>
 
using namespace std;
 
int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
 
    string target;
    cin >> target;
 
    string cur = "";
 
    for (int i = 1; i <= 100000; i++) {
        cur += to_string(i);
 
        if (cur.size() > target.size()) {
            cout << -1;
            return 0;
        }
 
        if (target.substr(0, cur.size()) != cur) {
            cout << -1;
            return 0;
        }
 
        if (cur == target) {
            cout << i;
            return 0;
        }
    }
 
    cout << -1;
 
    return 0;
}
 

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