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1038.cpp

#include <iostream>
#include <list>
#include <map>
#include <unordered_map>
using namespace std;

unordered_map<int, list<long>> m;
int N;

long solution() {
    if(N <= 9) return N;
    N -= 9;
    int numberOfDigits = 2;

    while(numberOfDigits <= 10) {
        list<long> prevComb = m[numberOfDigits - 1];
        list<long> curComb;
        for(auto num : prevComb) {
            int endOfDigit = num % 10;
            for(int i = 0; i < endOfDigit; i++) {
                curComb.push_back(num * 10 + i);
                N--;
                if(!N) return num * 10 + i;
            }
        }
        m[numberOfDigits++] = curComb;
    }
    return -1;
}

int main() {
    m[1] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
    cin>>N;

    cout<<solution();

}