#include <iostream>
#include <limits>

char ch;
bool error_c = 0;

void Read();
void Expression();
void Radical();
void Natural();
void Number();
void TryRead();
void Error();

int main() {
    while (true) {
        TryRead();
        if (!error_c) {
            std::cout << "Parsing complete!\n";
        }
    }
    return 0;
}

void Expression() {
    if (error_c) return;
    if ((ch == 'V') || (ch == '{')) {
        Radical();
        Expression();
    }
    else if (ch == '(') {
        Read();
        Expression();
        if ((ch == '+') || (ch == '-')) Read(); else { Error(); return;}
        Expression();
        if (ch == ')') Read(); else { Error(); return; }
    }
    else if ((ch > '0') && (ch <= '9')) {
        Natural();
    } else {Error(); return;}
}

void Radical() {
    if (ch == 'V') Read();
    else if (ch == '{') {
        Read();
        Natural();
        if (ch == '}') Read(); else {Error(); return;}
        if (ch == 'V') Read(); else {Error(); return;}
    } else Error();
}

void Natural() {
    if ((ch > '0') && (ch <= '9')) Read(); else Error();
    Number();
}

void Number() {
    if ((ch >= '0') && (ch <= '9')) {
        Read();
        Number();
    }
}

void Error() {
    std::cout << "Error parsing expression!\n";
    std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
    std::cin.clear();
    error_c = 1;
}

void TryRead() {
    error_c = 0;
    std::cout << "Enter expression (it should end with '.' e.g. \"1234.\" is valid expression): ";
    std::cin >> ch;
    Expression();
}

void Read() {
    std::cin >> ch;
}