java if statement check not working : Whenever I run this code with a negative number for the first int it keeps running the code even though I have an if statement that checks for this.

    import java.util.Scanner;
	public static void main(String[] args) {
		int a,b,c;
		double r1, r2, d;
		Scanner s = new Scanner(System.in);
		System.out.println("Enter the coefficients of the equation: ");
		System.out.println("Enter a: ");
		a = s.nextInt();
		System.out.println("Enter b: ");
		b = s.nextInt();
		System.out.println("Enter c: ");
		c = s.nextInt();
		if(a <= 0) {
			System.out.println("Error: " +"'a'" +" cannot be less than 1");
		}
		else {
			
		}
		System.out.println("Given dquadratic equation:" + a + "x^2 + " + b + "x + " + c + ",");
		d = b * b - 4 * a * c;
		if(d > 0) {
			System.out.println("Roots are real and unequal");
			r1 = (-b + Math.sqrt(d)/(2*a));
			r2 = (-b + Math.sqrt(d)/(2*a));
			System.out.println("Root 1: " + r1);
			System.out.println("Root 2: " + r2);
		}
		else if(d == 0){
			System.out.println("Roots are real and equal");
			r1 = (-b + Math.sqrt(d)/(2*a));
			System.out.println("Root 1: " + r1);
		}
		else {
			System.out.println("Roots are imaginary");
		}
	}

}