Codeforces Round #177 (Div. 1) A - Polo the Penguin and Strings

#include <iostream>
#include <queue>
#include <stack>
#include <map>
#include <set>
#include <bitset>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <climits>
#include <cstdlib>
#include <cmath>
#include <time.h>
#define maxn 1000005
#define maxm 2000005
#define eps 1e-7
#define mod 1000000007
#define INF 0x3f3f3f3f
#define PI (acos(-1.0))
#define lowbit(x) (x&(-x))
#define mp make_pair
#define ls o<<1
#define rs o<<1 | 1
#define lson o<<1, L, mid
#define rson o<<1 | 1, mid+1, R
#define pii pair<int, int>
#pragma comment(linker, "/STACK:16777216")
typedef long long LL;
typedef unsigned long long ULL;
using namespace std;
LL qpow(LL a, LL b){LL res=1,base=a;while(b){if(b%2)res=res*base;base=base*base;b/=2;}return res;}
LL powmod(LL a, LL b){LL res=1,base=a;while(b){if(b%2)res=res*base%mod;base=base*base%mod;b/=2;}return res;}
//head

int n, k;
char ans[maxn];

void read()
{
scanf("%d%d", &n, &k);
}

void work()
{
if(n < k || (k == 1 && n > 1)) {
printf("-1\n");
return;
}
for(int i = 1; i <= n; i++) {
if(i % 2) ans[i] = 'a';
else ans[i] = 'b';
}
for(int i = n; i > n - k + 2; i--)
ans[i] = k + i - n  - 1+ 'a';
for(int i = 1; i <= n; i++) printf("%c", ans[i]);
printf("\n");
}

int main()
{
read();
work();

return 0;
}