Sum within Range

You are initially given an array of $N$ integers. Given this array, you have to perform 2 kinds of operations: $1$ $L$ $R :$ You are given 2 integers $L$ and $R$. You need to return the sum of all the elements with indices between $L$ and $R$ ( both inclusive ). That is, if the elements currently in the array are a1, a2, a3.... an, you need to return the following sum : aL + aL+1 + aL+2 ... + aR.