Mechanism of a hypothetical reaction $X _{2}+ Y _{2} \rightarrow 2 XY$ is given below:
(i) $X_{2} \rightarrow X+X$ (fast)
(ii) $X + Y _{2} \rightleftharpoons XY + Y$ ( slow )
(iii) $X + Y \rightarrow XY$ (fast)
The overall order of the reaction will be