#include <iostream>
#include <memory>
#include <string>

class Expression {
public:
    virtual int Evaluate() const = 0;
    virtual std::string ToString() const = 0;
    virtual ~Expression() {}
};

using ExpressionPtr = std::shared_ptr<Expression>;

class Const: public Expression {
private:
    ExpressionPtr left = nullptr;
    ExpressionPtr right = nullptr;
    int value = 0;

public:
    Const(const int& _val) : value(_val) {}

    int Evaluate() const override {
        return value;
    }

    std::string ToString() const override {
        return std::to_string(value);
    }
};

class Sum : public Expression {
private:
    ExpressionPtr left = nullptr;
    ExpressionPtr right = nullptr;

public:
    Sum(const ExpressionPtr& _left, const ExpressionPtr& _right)
    : left(_left), right(_right) {
    }

    int Evaluate() const override {
        return (left->Evaluate() + right->Evaluate());
    }

    std::string ToString() const override {
        std::string temp;
        temp += left->ToString();
        temp += " + ";
        temp += right->ToString();
        return temp;
    }
};

ExpressionPtr Const(const int& val) {
    std::shared_ptr<Expression> sh(new class Const(val));
    return sh;
}

ExpressionPtr Sum(const ExpressionPtr& ex1, const ExpressionPtr& ex2) {
    std::shared_ptr<Expression> sh(new class Sum(ex1, ex2));
    return sh;
}