#include<cstdio>
#include<cmath>
#include<string.h>
#define ll long long 
#define N 35

using namespace std;

char ch[N];

ll n, l;

int main(void)
{
	scanf("%s", ch+1);  // 从下标1位置开始输入
	l = strlen(ch+1); // 求出输入字符的长度 
	scanf("%lld", &n);
	ll low, k, now = n;
	
	while(now > l){
		
		// 计算now位置的对应下界 
		k = floor(log2(now/l)); // 向下取整
		low = l*(long long)(pow(2,k));
		
		if(now == low + 1){
			now = 1;
		}else
			now = (now - low - 1);
	}
	printf("%c", ch[now]); 
	return 0;
}

上面代码能A，但是被hack：

COW 12