Millis Issue

Projects General Guidance
4

Couldn't you come up with better names for f and b, and what's their purpose, really?
It seems you use them as flag - while it's much easier to use a single boolean flag and just check whether it's true or false. Like this:

bool f = false;

[...]

//  if((digitalRead(A0) == HIGH) && f==b) // if(digitalRead(A0)) or if(!digitalRead(A0))
  if((digitalRead(A0) == HIGH) && !f) // if(digitalRead(A0)) or if(!digitalRead(A0))
  {
    timer = millis();
//    f = f+1;
    f = true;
  }
//  if((!digitalRead(A0))&& f==b+1)
  if((!digitalRead(A0)) && f)
  {
    timef = millis() - timer;
//    f = f-1;
    f = false;
    timenow = millis();
  }

Now give f a more descriptive name and your code has become a lot more readable. Also do a CTRL-T in the IDE to fix your indentation.