Another way array entered in B1 and filled down. If you aren’t familiar with array-entered formulas array enter means the formula must be committed from edit mode by simultaneously pressing and holding down Ctrl and Shift while hitting Enter.
Formula: copy to clipboard 
=IFERROR(IF(MATCH(ROWS($1:1),IF($A$1:$A$17="Start",(ROW($A$1:$A$17)-1)+$C$1),0),"End"),"")