289C - Polo the Penguin and Strings - CodeForces Solution

#include <bits/stdc++.h>
#define int long long
using namespace std;

int32_t main(){
	int n, k;
	cin >> n >> k;
	string s  = "";
	if(k > n) cout << -1 << endl;
	else if(k == 1 && n != 1) cout << -1 << endl;
	else if(k == n) {
		for(int i = 0; i < k; i++) s += char(i + 'a');
		cout << s << endl;
	}
	else{
		for(int i = 0; i < n - k + 2; i++){
			if(i % 2 == 0) s += 'a';
			else s += 'b';
		}
		for(int i = 0; i < k - 2; i++){
			s += char(i + 'c');
		}
		cout << s << endl;
	}
}