t = int(input())
for i in range(t):
ans = list()
a, s = map(str, input().split())
j = len(a) - 1
k = len(s) - 1
lis = ''
while True:
if k < j:
ans = [-1]
break
if j == -1:
lis = s[:k + 1]
break
if s[k] < a[j] and k > 0:
if s[k - 1] == '1':
s1 = int(s[k - 1] + s[k])
k -= 1
ans.append(s1 - int(a[j]))
else:
ans = [-1]
break
elif s[k] < a[j] and k == 0:
ans = [-1]
break
else:
ans.append(int(s[k]) - int(a[j]))
k -= 1
j -= 1
if len(lis) == 0:
while ans[-1] == 0 and len(ans) > 1:
ans.pop()
ans = reversed(ans)
for i in ans:
lis += str(i)
print(lis)#include <bits/stdc++.h>
using namespace std;
#define ll long long
int main(){
int t; cin>>t;
while(t--){
ll a,s; cin>>a>>s;
bool flag=true;
ll e=1,b=0;
while(s>0){
ll d=a%10,f=s%10;
s/=10,a/=10;
ll w=10;
if(f<d){
f=f+w*(s%10);
s/=10,w*=10;
}
if(f-d>9 || f<d){
flag=false;
break;
}
b+=e*(f-d);
e*=10;
}
if(a!=0){
flag=false;
}
if(flag){
cout<<b<<endl;
}
else{
cout<<-1<<endl;
}
}
return 0;
}441C - Valera and Tubes | 1328E - Tree Queries |
265A - Colorful Stones (Simplified Edition) | 296A - Yaroslav and Permutations |
967B - Watering System | 152A - Marks |
1398A - Bad Triangle | 137A - Postcards and photos |
1674D - A-B-C Sort | 334A - Candy Bags |
855A - Tom Riddle's Diary | 1417A - Copy-paste |
1038A - Equality | 1061A - Coins |
1676E - Eating Queries | 1447A - Add Candies |
1721D - Maximum AND | 363C - Fixing Typos |
1401A - Distance and Axis | 658A - Bear and Reverse Radewoosh |
1721E - Prefix Function Queries | 977E - Cyclic Components |
1140D - Minimum Triangulation | 75C - Modified GCD |
1722A - Spell Check | 1722B - Colourblindness |
1722D - Line | 1722C - Word Game |
1722G - Even-Odd XOR | 552E - Vanya and Brackets |