how to get averarage from hashmap?

import java.util.HashMap;
import java.util.Map;

class Student {
            String name;
            String surname;
            String id;

            public Student(String name, String surname, String id) {
                this.name = name;
                this.surname = surname;
                this.id = id;
            }

            @Override
            public boolean equals(Object o) {
                Student e = (Student) o;
                return this.name.equals(e.name) && this.surname.equals(e.surname) && this.id.equals(e.id);
            }

            @Override
            public int hashCode() {
                return Integer.parseInt(id.substring(0, 3));
            }
        }

    class Grades  {
        int grade1;
        int grade2;
        int grade3;
        int grade4;
        int grade5;
        int grade6;
        int grade7;

        public Grades(int grade1, int grade2, int grade3, int grade4, int grade5, int grade6, int grade7) {
            this.grade1 = grade1;
            this.grade2 = grade2;
            this.grade3 = grade3;
            this.grade4 = grade4;
            this.grade5 = grade5;
            this.grade6 = grade6;
            this.grade7 = grade7;
        }
        @Override
        public String toString() {
            return grade1 + ", " + grade2 + ", " +  grade3 + ", " +  grade4 + ", " +  grade5 + ", " +  grade6 + ", " +  grade7;
        }
    }
public class ObjectsTestetON {
    public static void main(String[] args) {
        Student student1 = new Student("Ania", "Nowak", "00001");
        Student student2 = new Student("Marcin", "Kasprzak", "00002");
        Student student3 = new Student("Kasia", "Siodłak", "00003");
        Student student4 = new Student("Błażej", "Markowski", "00004");

        Grades grad1 = new Grades(5, 4, 5, 4, 5, 5, 4);
        Grades grad2 = new Grades(3, 4, 5, 4, 3, 4, 4);
        Grades grad3 = new Grades(4, 4, 5, 4, 4, 5, 4);
        Grades grad4 = new Grades(2, 4, 3, 4, 3, 2, 3);

        Map<Student, Grades> gradesParameter = new HashMap<>();

        gradesParameter.put(student1, grad1);
        gradesParameter.put(student2, grad1);
        gradesParameter.put(student3, grad1);
        gradesParameter.put(student4, grad1);



        for (Grades i : gradesParameter.keySet()) {
            System.out.println("key: " + i + " value: " + gradesParameter.get(i));
        }
    }
    }