#include <bits/stdc++.h>   using namespace std;   int main() {     int n, k;     cin >> n >> k;     if (k > n) {       cout <<  -1 ;       return 0;     }     if (n == 1) {       cout <<  a ;       return 0;     }     if (n > 1 && k == 1) {       cout <<  -1 ;       return 0;     }     string str;     char c1 =  a , c2 =  b ;     for (int i = 0; i < n; i++) {       str += c1;       if (i % 2 == 0)         c1 =  b ;       else         c1 =  a ;     }     char ch =  a  + k - 1;     int l = str.size() - 1;     for (int i = 0; i < k - 2; i++) str[l--] = ch--;     cout << str;     return 0;   }   