Sheldon and Leonard are physicists who are fixated on the BIG BANG theory. In order to exchange secret insights they have devised a code that encodes UPPERCASE words by shifting their letters forward.
谢耳朵和莱纳德是研究BB理论的物理学家。要用暗号（大写字母）联系。
Shifting a letter by S positions means to go forward S letters in the alphabet. For example, shifting B by S = 3 positions gives E. However, sometimes this makes us go past Z, the last letter of the alphabet. Whenever this happens we wrap around, treating A as the letter that follows Z. For example, shifting Z by S = 2 positions gives B.
（读懂这段话是解题的关键，翻译了就没意义了）
Sheldon and Leonard’s code depends on a parameter K and also varies depending on the position of each letter in the word. For the letter at position P, they use the shift value of S = 3P + K.
他们有一个密钥K。第P个字母有S = 3P + K。
For example, here is how ZOOM is encoded when K = 3. The first letter Z has a shift value of S = 3 × 1 + 3 = 6; it wraps around and becomes the letter F. The second letter, O, has S = 3 × 2 + 3 = 9 and becomes X. The last two letters become A and B. So Sheldon sends Leonard the secret message: FXAB
Write a program for Leonard that will decode messages sent by Sheldon.
The input will be two lines. The first line will contain the positive integer K (K < 10), which is used to compute the shift value. The second line of input will be the word, which will be a sequence of uppercase characters of length at most 20.
输入有两行。第一行一个正整数K（〈10）。第二行是最多20个大写字母组成的信。
The output will be the decoded word of uppercase letters.
输入就是解密后的信。
样例1：
3
FXAB
样例2:
5
JTUSUKG
样例1：
ZOOM
样例2：
BIGBANG
#include<algorithm>
#include<iostream>
#include<cstring>
#include<cstdio>
#include<cmath>
using namespace std;
int main()
{
char b[25];
int i,k,s;
scanf("%d",&k);
scanf("%s",b);
for(i=0;b[i];i++)
{
s=3*(i+1)+k;
if(b[i]-s<'A')
{
s-=b[i]-'A';
b[i]='Z'+1;
}
printf("%c",b[i]-s);
}
}
