In this example the number of remaining chars is 20...
    
    F  L  A  M  E  S
    1  2  3  4  5  6
    7  8  9  10 11 12
    13 14 15 16 17 18
    19 20               remove the L
    
    F  A  M  E  S
       1  2  3  4       now start at A
    ..
    20                  remove the F
    
    A  M  E  S
    1  2  3  4          now start at A
    ..
    17 18 19 20         remove the S
    
    A  M  E
    1  2  3             now start at A
    ..
    19 20               remove the M
    
    A  E
       1                now start at E
    ..
    20                  remove the A
    
    E                   this is the key