hristomir

#include <iostream>
#include <vector>
#include <string>
#include <cctype>

bool isValidNumber(std::string& n) {
    return (isdigit(n[0]) || ((n[0] == '-') && isdigit(n[1])));
}

void handleSum(std::vector<int>& numbers) {
    int lastNum = numbers[numbers.size() - 1];
    int secondLast = numbers[numbers.size() - 2];
    numbers.pop_back();
    numbers.pop_back();
    numbers.push_back(lastNum + secondLast);
}

void handleSubtract(std::vector<int>& numbers) {
    int lastNum = numbers[numbers.size() - 1];
    int secondLast = numbers[numbers.size() - 2];
    numbers.pop_back();
    numbers.pop_back();
    numbers.push_back(lastNum - secondLast);
}

void handleConcat(std::vector<int>& numbers) {
    int lastNum = numbers[numbers.size() - 1];
    int secondLast = numbers[numbers.size() - 2];
    numbers.pop_back();
    numbers.pop_back();
    std::string firstNum = std::to_string(secondLast);
    std::string secondNum = std::to_string(lastNum);
    std::string result = firstNum + secondNum;
    numbers.push_back(std::stoi(result));
}

void discard(std::vector<int>& numbers) {
    numbers.pop_back();
}

int main() {
    std::vector<int> numbers;
    std::string input;
    while (std::cin >> input && input != "end") {
        if (isValidNumber(input)) {
            numbers.push_back(std::stoi(input));
            continue;
        }
        if (input == "sum") handleSum(numbers);
        else if (input == "subtract") handleSubtract(numbers);
        else if (input == "concat") handleConcat(numbers);
        else if (input == "discard") discard(numbers);
    }

    for (auto& n : numbers) std::cout << n << std::endl;
}