#include <iostream>
#include <limits>
#include <string>
#include <sstream>
 
int GetMark()
{
   std::cout << "enter mark (0 - 100): " << std::endl;
 
   std::string line;
   while ( getline(std::cin, line) )
   {
      std::istringstream str(line);
      int mark;
      if ( str >> mark )
      {
         if (  mark >= 0 && mark <= 100)
         {
            return mark;
         }
      }
      std::cout << "Invalid input: " << line << std::endl;
      std::cout << "enter mark (0 - 100): " << std::endl;
   }
 
   // Unable to read.
   // Throw exception, or exit with an error message.
   return 0;
}
 
int main()
{
   int mark = GetMark();
   std::cout << "mark: " << mark << std::endl;
}
 
 

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QmFkIGlucHV0CkJhZCBpbnB1dCAyCjEwMjAyMAotMjAyMDIwMgo4NQ==

Bad input
Bad input 2
102020
-2020202
85