Army

### 题目描述 在北爱尔兰军队的行政系统中有 $n$ 个军衔，其中 $1$ 最低，$n$ 最高。 你需要 $d_i$ 年来从 $i$ 级升到 $i+1$ 级。在你升到 $i$ 级时必须已经到达前面所有等级。 Vasya 刚刚达到 $a$ 级，但他想达到 $b$ 级。请问他至少还要再参军多少年。 ### 输入格式 第一行：$n$（$2 \leq n \leq 100$）； 第二行：$d_1$，$d_2$，$d_3……d_{n-1}$（$1 \leq d_i \leq 100$）； 第三行：$a,b$（$1$ $\leq$ $a$ $<$ $b$ $\leq$ $n$）。 ### 输出格式 仅一行：从 $a$ 级升到 $b$ 级所需的年数。

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 $ .

3
5 6
1 2

5

3
5 6
1 3

11