CODE EXAMPLE FOR CPP

how to use string in if else statement c++

double const FRAME_COST = 45.00;
string input;
char yes,
     no;
int frames;


cout << "Do you want additional frames? Type yes/no:  ";
cin  >> input;

    if (input == yes){
       cout << "How many?"
       cin >> frames;
       frames = frames * FRAME_COST;
       }

// The problem is in **the if statement**
// I need to use a string not a bool (according to my professor)
// I can't get the compiler to recognize the **if statement**
// I realize this isn't practical, but he always throws curve balls.