import math
for _ in range(int(input())):
n,k=map(int,input().split())
y=0
if n<bin(k).count('1') or k<n:
y=-1
else:
k=bin(k)[2:]
k=k[::-1]
a=[]
for i in range(len(k)):
if k[i]=='1':
a.append(i)
for i in range(len(a)):
a[i]=2**a[i]
if y!=-1:
z=len(a)
j=0
for i in range(n-z):
if a[j]%2==0:
a[j]=int(a[j]/2)
a.append(a[j])
else:
j+=1
if a[j]%2==0:
a[j]=int(a[j]/2)
a.append(a[j])
for i in range(len(a)):
a[i]=int(math.log(a[i],2))
a[i]=a[i]+97
a[i]=chr(a[i])
x=''
for i in a:
x=x+i
print(x)
else:
print(-1)