import java.io.IOException;
import java.math.BigDecimal;
import java.util.HashSet;
import java.util.InputMismatchException;

public class Main {
	long divide(long n, long m) {

		if (n % m == 0) {
			long k = n / m;
			while (k % 10 == 0) {
				k /= 10;
			}
			return k % 10;
		}

		HashSet<Long> set = new HashSet<Long>();

		long a = n / m;
		long d = n % m;
		long ret = -1;
		while (true) {
			if (d < 0) {
				return 10;
			}
			if (d == 0) {
				break;
			}
			if (set.contains(d)) {
				ret = -1;
				break;
			}
			set.add(d);

			n = d * 10;
			a = n / m;
			d = n % m;
			ret = a;
		}

		return ret;
	}

	void run() {
		MyScanner sc = new MyScanner();

		long n = sc.nextLong();
		long m = sc.nextLong();

		if (n % m == 0) {
			if (n % m == 0) {
				long k = n / m;
				while (k % 10 == 0) {
					k /= 10;
				}
				System.out.println(k % 10);
			}
		} else {
			try {
				BigDecimal a = new BigDecimal("" + n).divide(new BigDecimal("" + m));
				char[] c = a.toPlainString().toCharArray();
				System.out.println(c[c.length - 1]);
			} catch (Exception e) {
				System.out.println(-1);
			}
		}
	}

	public static void main(String[] args) {
		new Main().run();
	}

	public void mapDebug(int[][] a) {
		System.out.println("--------map display---------");
		for (int i = 0; i < a.length; i++) {
			for (int j = 0; j < a[i].length; j++) {
				System.out.printf("%3d ", a[i][j]);
			}
			System.out.println();
		}
		System.out.println("----------------------------" + '\n');
	}

	class MyScanner {
		int read() {
			try {
				return System.in.read();
			} catch (IOException e) {
				throw new InputMismatchException();
			}
		}

		boolean isSpaceChar(int c) {
			return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
		}

		boolean isEndline(int c) {
			return c == '\n' || c == '\r' || c == -1;
		}

		int nextInt() {
			return Integer.parseInt(next());
		}

		int[] nextIntArray(int n) {
			int[] array = new int[n];
			for (int i = 0; i < n; i++)
				array[i] = nextInt();
			return array;
		}

		long nextLong() {
			return Long.parseLong(next());
		}

		long[] nextLongArray(int n) {
			long[] array = new long[n];
			for (int i = 0; i < n; i++)
				array[i] = nextLong();
			return array;
		}

		double nextDouble() {
			return Double.parseDouble(next());
		}

		double[] nextDoubleArray(int n) {
			double[] array = new double[n];
			for (int i = 0; i < n; i++)
				array[i] = nextDouble();
			return array;
		}

		String next() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			StringBuilder res = new StringBuilder();
			do {
				res.appendCodePoint(c);
				c = read();
			} while (!isSpaceChar(c));
			return res.toString();
		}

		String[] nextStringArray(int n) {
			String[] array = new String[n];
			for (int i = 0; i < n; i++)
				array[i] = next();

			return array;
		}

		String nextLine() {
			int c = read();
			while (isEndline(c))
				c = read();
			StringBuilder res = new StringBuilder();
			do {
				res.appendCodePoint(c);
				c = read();
			} while (!isEndline(c));
			return res.toString();
		}
	}
}