I am having trouble understanding why my program is giving me an error?

public static void main(String[] args) {
int agrades = 0;
int fgrades=0;
int count=0;
Scanner input = new Scanner(System.in);
System.out.println("Enter Grades: ");
int grades = input.nextInt();
while (grades>=0 && grades<=0){
if (grades>=90 && grades<=100)
        agrades++;
if  (grades>=0 && grades<=60)
        fgrades++;
count++;
grades = input.nextInt();
}
System.out.println("Amount of Grade: " + count);
System.out.println("Average Grade: " + (grades+grades/count));
System.out.println("Number of grades above 90: " + agrades);
System.out.println("Number of grades below 60: " + fgrades);
}
}

#Exception in thread "main" java.lang.ArithmeticException: / by zero
at javaapplication12.JavaApplication12.mainsnippets/run.xml:53: Java     returned: 1
BUILD FAILED (total time: 3 seconds)