#include<iostream>#include<cstdio>#include<string>#include<cstring>#include<algorithm>using namespace std;const int maxn = 200005;int n,k,a[maxn];char cmd;int main(){    freopen("str.in","r",stdin);    freopen("str.out","w",stdout);    ios::sync_with_stdio(false);    cin>>n>>k;    if(k == 1 && n == 1){        cout<<"a";        return 0;    }     if(k == 1 || k > n){        cout<<-1;        return 0;    }    int cnt = n;    for(int i = k;i >= 3;i--){        a[cnt] = i;        cnt--;    }    for(int i = 1;i <= cnt;i++){        if(i&1) a[i] = 1;        else a[i] = 2;    }    for(int i = 1;i <= n;i++){        cmd = ‘a‘ - 1 + a[i];        cout<<cmd;    }    return 0;}