A7929. Army

The Berland Armed Forces System consists of $n$ ranks that are numbered using natural numbers from $1$ to $n$ , where $1$ is the lowest rank and $n$ is the highest rank.

One needs exactly $d_{i}$ years to rise from rank $i$ to rank $i+1$ . Reaching a certain rank $i$ having not reached all the previous $i-1$ ranks is impossible.

Vasya has just reached a new rank of $a$ , but he dreams of holding the rank of $b$ . Find for how many more years Vasya should serve in the army until he can finally realize his dream.

The first input line contains an integer $n$ ( $2<=n<=100$ ). The second line contains $n-1$ integers $d_{i}$ ( $1<=d_{i}<=100$ ). The third input line contains two integers $a$ and $b$ ( $1<=a<b<=n$ ). The numbers on the lines are space-separated.

Print the single number which is the number of years that Vasya needs to rise from rank $a$ to rank $b$ .