function solveClasses() {
    class Person {
        constructor(firstName, lastName) {
            this.firstName = firstName;
            this.lastName = lastName;
            this.problems = [];
        }

        toString() {
            return `${this.firstName} ${this.lastName} is part of SoftUni community now!`
        }
    }

    class Teacher extends Person {
        constructor(firstName, lastName) {
            super(firstName, lastName);
        }

        createProblem(id, difficulty) {
            let problem = this.problems.find(x => x.id === id);

            if (!problem) {
                problem = {
                    id,
                    difficulty: Number(difficulty)
                }
                this.problems.push(problem);

            }
            return this.problems;
        }

        getProblems() {
            return this.problems;
        }

        showProblemSolution(id) {
            let problem = this.problems.find(x => x.id === id);

            if (!problem) {
                throw new Error(`Problem with id ${id} not found.`);
            } else {
                problem.difficulty--;
                return problem;
            }

        }

    }

    class Student extends Person{
        constructor( firstName, lastName, graduationCredits, problems ){
            super(firstName,lastName);
            this.graduationCredits = Number(graduationCredits);
            this.myCredits = 0;
            this.solvedProblems = [];
            this.problems = problems;
        }

        solveProblem(id){
            let problem = this.problems.find(x => x.id === id);

            if(!problem){
                throw new Error(`Problem with id ${ id } not found.`);
            } else{
                if(!this.solvedProblems.includes(problem)){
                    this.myCredits += problem.difficulty;
                    this.solvedProblems.push(problem);
                }
            }
            return this.myCredits;
        }

        graduate(){
            if(this.myCredits >= this.graduationCredits){
                return `${this.firstName} ${this.lastName} has graduated succesfully.`;
            } else{
                let neededCredits = this.graduationCredits - this.myCredits;
                return `${this.firstName} ${this.lastName}, you need ${ neededCredits } credits to graduate.`;
            }
        }
    }

    return {
        Person,
        Teacher,
        Student
    }
}