Tried my first real codeforces round, got stuck way too long on one of the earlier questions, but over all, you gotta start somewhere i guess

codeforces/rounds/762/c.cpp

@@ -0,0 +1,72 @@
+#include <bits/stdc++.h>
+
+using namespace std;
+
+typedef long long ll;
+
+int first_digit(ll& n){
+    int digits = (int)log10(n);
+
+    n = (int)(n / pow(10, digits));
+
+    return n;
+}
+
+void test_case(int tc){
+    ll a, s;
+    cin >> a >> s;
+
+    ll b = 0;
+    int bpow = 0;
+
+    while(a > 0){
+        if(s == 0){
+            cout << -1 << '\n';
+            return;
+        }
+
+        int la = a % 10, ls = s % 10;
+        if(ls < la){
+            ls = s % 100;
+            s /= 10;
+        }
+
+        if(ls - la > 9 || la > ls){
+            cout << -1 << '\n';
+            return;
+        }
+
+        a /= 10;
+        s /= 10;
+
+        b += (ll)pow(10, bpow) * (ls - la);
+        bpow++;
+    }
+
+    while(s > 0){
+        b += (ll)pow(10, bpow) * (s % 10);
+        s /= 10;
+        bpow++;
+    }
+
+    cout << b << '\n';
+    return;
+}
+
+
+
+int main(){
+    ios::sync_with_stdio(0);
+    cin.tie(0);
+
+    int t;
+    cin >> t;
+
+    for(int tc = 1; tc <= t; ++tc){
+        test_case(tc);
+    }
+
+    cout << flush;
+
+    return 0;
+}