If $X$ and $Y$ are discrete random variables and $X\leq Y$, then $E(X)\leq E(Y)$

If $X$ and $Y$ are discrete random variables and $X\leq Y$, then $E(X)\leq
E(Y)$

How to prove this kind of question?
Let $X$ and $Y$ be discrete random variables defined on sample probability
space. If $X\leq Y$, then $E(X)\leq E(Y)$.