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;
}796A - Buying A House | 67A - Partial Teacher |
116A - Tram | 1472B - Fair Division |
1281C - Cut and Paste | 141A - Amusing Joke |
112A - Petya and Strings | 677A - Vanya and Fence |
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832A - Sasha and Sticks | 292B - Network Topology |
1339A - Filling Diamonds | 910A - The Way to Home |
617A - Elephant | 48A - Rock-paper-scissors |
294A - Shaass and Oskols | 1213A - Chips Moving |
490A - Team Olympiad | 233A - Perfect Permutation |
1360A - Minimal Square | 467A - George and Accommodation |