Unexpected syntax error in IDL when redefining using if, then and endif

Getting an unexpected syntax error message when trying to redefine constants using if, then and endif

if(ipeak eq A3(n3-1)) then begin
;if wave packets too close then need to redefine halfl and half: 
        if(A3(ipeak)-A3(ipeak-1) lt 2*half) then 
        halfl=(A3(ipeak)-A3(ipeak-1))/2.
        if (half gt nn-A3(n3-1) )then halfr=nn-A3(n3-1)
endif 

With the syntax error occurring on the second if statement at the end over the then statement. Am I being stupid or have been staring at this too long because I cannot see the issue properly