Having trouble with visual stuio 2019

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C++

Visual-Studio

VisualC++

I just started using Visual Studio for the first time. I wanted to try and use my knowledge of C++ to build a simple windows application. The application is supposed to calculate a simple, one expression derivative such as 4x^2, 2x^3, 24x^5 etc. (I was going to make it do more but I had to make sure this worked first) However, it doesn't seem to work (at least not on Visual Studio).

What I have tried:

This is the code for the button that the user will press to calculate. The answer is supposed to be displayed in label4:

private: System::Void button1_Click(System::Object^ sender, System::EventArgs^ e) {

		std::string expression;
		msclr::interop::marshal_context context;
		expression = context.marshal_as<std::string>(textBox1->Text);

		std::string firstNum;
		std::string firstExpr;

		for (int i = 0; i < expression.size(); ++i)
		{
			if (isdigit(expression.at(i)) == true)
			{
				firstNum = firstNum + expression.at(i);
			}
			else
			{
				int num = atoi(firstNum.c_str());
				std::string powNum;
				powNum = powNum + expression.at(i + 2);
				int pow = atoi(powNum.c_str());
				num = num * pow;
				--pow;
				firstExpr = std::to_string(num) + "x^";
				firstExpr = firstExpr + std::to_string(pow);
				break;
			}
		}
		label4->Text = context.marshal_as<String^>(firstExpr);
	
	}

at first I used stoi() instead of atoi() but I kept getting an exception thrown saying stoi(out of range). I Copied and pasted the code to Notepad++ (with very minor tweaks) and compiled it on the GNU compiler through the Putty terminal and it worked just fine. I gave up with using stoi() and tried using atoi() and c_str() to first turn the string into a c string then convert it to an int. However, although I no longer get an exception, the output is always 0x^-1. I tried as much as possible to use the C++ stuff I am familiar with (hence why I did everything using C++ strings instead of Microsoft's String ^) since I am new to this but It's just not working in Visual Studio for some reason. Could I please get some help with this, I would greatly appreciate it. Thank you.