import java.util.Scanner;

public class ProblemA {

	public static void main(String[] args) {
		int a, b, c, d, x;
		Scanner s = new Scanner(System.in);

		while (true) {
			a = s.nextInt();
			b = s.nextInt();
			c = s.nextInt();
			d = s.nextInt();
			if (a == -1 && b == -1 && c == -1 && d == -1) break;
			x = solve(a, b, c, d);
			System.out.println(x);
		}

	}

	private static int solve(int a, int b, int c, int d) {

		if (a == -1) {
			if (d - c == c - b && isInRange(b - (d - c))) return b - (d - c);	//Arithmetic test
			else if (d % c == 0 && c % b == 0 && d/c == c/b && b % (d/c) == 0 && isInRange(b/(d/c))) return b/(d/c); //Geometric test
			else return -1;
		} else if (b == -1) {
			if (2*(d - c) == c - a && isInRange(a + (d - c))) return a + (d - c);
			else if (d % c == 0 && c % a == 0 && (d/c)*(d/c) == (c/a) && isInRange(a*(d/c))) return a*(d/c);
			else return -1;
		} else if (c == -1) {
			if (d - b == 2*(b - a) && isInRange(b + (b - a))) return b + (b - a);
			else if (d % b == 0 && b % a == 0 && (b/a)*(b/a) == (d/b) && isInRange(b*(b/a))) return b*(b/a);
			else return -1;
		} else { // d == -1
			if (c - b == b - a && isInRange(c + (b - a))) return c + (b - a);
			else if (c % b == 0 && b % a == 0 && c/b == b/a && isInRange(c*(b/a))) return c*(b/a);
			else return -1;
		}
	}

	private static boolean isInRange(int x) {
		if (x >= 1 && x <= 1000000) return true;
		else return false;
	}

}