What is the complexity of the following algorithm

public int m$1($int x$[])\{$

int s$=0$;

$($int i$=$x$.$length$-1$;i$>=0$;i$=$i$/2)$

s$=$s$+$x$[$j$]$;

return s;

$\}$

O$($log n$)$

O$($n$2)$

O$(1)$

O$($n$)$