import java.util.Scanner;

public class Range {

	// s(x) vrakja zbir na cifrite na x
	static long s(long x) {
		int sumaCifri = 0;
		while (x != 0) {
			sumaCifri += x % 10;
			x = x / 10;
		}
		return sumaCifri;
	}


	static long proveri(long N, long A, long B) {
		// Vasiot kod tuka
		long middle = (A + B) / 2;

		if (A == B ) {
			if (A == B)
				{
				if(middle*middle+s(middle)+200==N)
					return middle;
				else
					return (long)-1;
				}


		} else {
			if (middle*middle+s(middle)+200 > N) {
				return proveri(N, A, middle);
			} else {
				return proveri(N, middle, B);
			}
		}
	}

	 public static void main(String[] args) throws Exception {
	        int i, j, k;
	        Scanner input = new Scanner(System.in);

	        long N = input.nextLong();

	        long A = input.nextLong();
	        long B = input.nextLong();

	        long res = proveri(N, A, B);
	        System.out.println(res);

	        input.close();

	    }

	}