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.

The first input line contains an integer $ n $ ( $ 2

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