#include <bits/stdc++.h>
using namespace std;
//
#define ll long long
#define ull unsigned long long
#define mx 100010
#define mod 1000000007
#define inf INT_MAX
#define pi acos(-1)
#define endl '\n'
#define pb push_back
#define pll pair<ll, ll>
#define Fast ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0)
//
int main() {
    ll t;
    cin >> t;

    while (t--) {
        string a, sum;
        cin >> a >> sum;

        sum = "0" + sum;
        ll j = a.length() - 1;
        vector<ll> vec;
        bool possible = true;
        for (ll i = sum.length() - 1; i >= 1; i--) {

            if (j == -1) {
                ll temp = sum[i] - '0';
                vec.pb(temp);
                continue;
            }

            ll tempA = a[j] - '0', tempSum = sum[i] - '0';

            if (a[j] == sum[i] && a[j] == '0') vec.pb(0);
            else if (tempA == tempSum) {
                vec.pb(0);
            }
            else if (tempSum > tempA) vec.pb(tempSum - tempA);
            else if (tempSum < tempA) {
                if (sum[i - 1] != '1') {
                    possible = false;
                    break;
                }
                vec.pb(10 + tempSum - tempA);
                i--;
            }
            j--;
        }

        if (!possible || j != -1) cout << -1 << endl;
        else {
            bool found = false;
            vector<ll>::iterator it = vec.end() - 1;
            for (it = vec.end() - 1; ; it--) {
                if (*it != 0) break;
                //cout << *it << endl;
                vec.erase(it);
                if (it == vec.begin()) break;
            }
            if (vec.size() == 0) {
                cout << 0 << endl;
                continue;
            }

            it = vec.end() - 1;
            for (; ; it--) {
                if (it == vec.end() - 1 && *it == 0) continue;
                cout << *it;
                if (it == vec.begin()) break;
            }
            cout << endl;
        }
    }
}