Codeforces Round #177 (Div. 1) / 288A Polo the Penguin and Strings(贪心)

A. Polo the Penguin and Strings

http://codeforces.com/problemset/problem/288/A

time limit per test

2 seconds

memory limit per test

256 megabytes

input

standard input

output

standard output

Little penguin Polo adores strings. But most of all he adores strings of length n.

One day he wanted to find a string that meets the following conditions:

1. The string consists of n lowercase English letters (that is, the string's length equals n), exactly k of these letters are distinct.
2. No two neighbouring letters of a string coincide; that is, if we represent a string as s = s₁s₂... s_n, then the following inequality holds,s_i ≠ s_i + 1(1 ≤ i < n).
3. Among all strings that meet points 1 and 2, the required string is lexicographically smallest.

Help him find such string or state that such string doesn't exist.

String x = x₁x₂... x_p is lexicographically less than string y = y₁y₂... y_q, if either p < q and x₁ = y₁, x₂ = y₂, ... , x_p = y_p, or there is such number r (r < p, r < q), that x₁ = y₁, x₂ = y₂, ... , x_r = y_r and x_r + 1 < y_r + 1. The characters of the strings are compared by their ASCII codes.

Input

A single line contains two positive integers n and k (1 ≤ n ≤ 10⁶, 1 ≤ k ≤ 26) — the string's length and the number of distinct letters.

Output

In a single line print the required string. If there isn't such string, print "-1" (without the quotes).

Sample test(s)

input

7 4

output

ababacd

input

4 7

output

-1

大水题。

完整代码：

/*30ms,0KB*/#include<cstdio>int main(){int n, k;scanf("%d%d", &n, &k);if (n < k)printf("-1");else if (n == k)for (int i = 97; i < 97 + k; ++i)putchar(i);else if (k == 1)printf("-1");else{n -= k - 2;if (n & 1){n = (n - 1) >> 1;for (int i = 0; i < n; ++i){putchar('a');putchar('b');}putchar('a');}else{n >>= 1;for (int i = 0; i < n; ++i){putchar('a');putchar('b');}}for (int i = 99; i < 97 + k; ++i)putchar(i);}return 0;}