Mechanism of a hypothetical reaction $X_2$ + $Y_2$ $\to$ 2XY is given below : (i) $X_2$ $\to$ X + X (fast) (ii) X + $Y_2$ $\stackrel{\longrightarrow}{\longleftarrow}$ XY + Y (slow) (iii) X + Y $\to$ XY (fast) The overall order of the reaction will be