CF38A.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.

俄罗斯联邦武装部队系统包含 $n$ 个军衔，这些军衔使用从 $1$ 到 $n$ 的自然数编号，其中 $1$ 为最低军衔，$n$ 为最高军衔。

从第 $i$ 级军衔晋升至第 $i+1$ 级军衔恰好需要 $d_i$ 年。若未依次获得前 $i-1$ 级军衔，则无法直接获得第 $i$ 级军衔。

瓦西娅刚刚晋升至第 $a$ 级军衔，但他梦想着能获得第 $b$ 级军衔。请计算瓦西娅还需在军队服役多少年，才能最终实现他的梦想。

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.

第一行输入包含一个整数 $n$（$2 \leq n \leq 100$）。第二行包含 $n-1$ 个整数 $d_i$（$1 \leq d_i \leq 100$）。第三行输入包含两个整数 $a$ 和 $b$（$1 \leq a < b \leq n$）。每行中的数字均以空格分隔。

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

输出唯一的数字，即瓦西娅从等级 $a$ 提升到等级 $b$ 所需的年数。