getline() reads unnecessary thing in if-statements, causes bad output (C++)? [duplicate]

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• Why does std::getline() skip input after a formatted extraction? 3 answers

I'm trying to finish up my "Math addition quiz" program but I have an issue with the output. Here's my code:

#include <iostream>
#include <cstdlib>
#include <string>

using namespace std;

int main() 
{
  string choice;

  do
  {
    cout << "Please type something: ";
    getline(cin, choice);
    while (choice != "1" && choice != "5")  //validation against string
      {
        cout << "Incorrect choice! Please input a valid number: >_";
        getline(cin, choice);
      } 

    unsigned seed = time(0);
    srand(seed);

    if (choice == "1")
    {
      double num1, num2,
      result, answer;

      num1 = rand() % 1000;
      num2 = rand() % 1000;

      result = num1 + num2;

      cout << "\n  " << num1 << " + " << num2 << " = ";          
      cin >> answer;

      if (answer == result)
        cout << "\nCongratulation! You got it right!\n\n";
      else
        cout << "\nNope! The correct answer is " << result << "\n\n";
    }
    else if (choice == "5")
      cout << "\nThank you for using the program!\n";
  } while (choice == "1"); 

  return 0;
}

After I press 1, and do the math (doesn't matter if I did it correctly or not), the second time output ends up as follow:

Please type something: Incorrect choice! Please input a valid number: >_

Please help me fix this. My guess is that 'getline' reads something in the nested if, I just don't know which one.