#include <bits/stdc++.h>
using namespace std;
template <typename A, typename B>
string to_string(pair<A, B> p);
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);
string to_string(const string& s) { return '"' + s + '"'; }
string to_string(const char& ch) {
  string s = "";
  s += ch;
  return "'" + s + "', ";
}
string to_string(const char* s) { return to_string((string)s); }
string to_string(bool b) { return (b ? "true" : "false"); }
string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}
template <size_t N>
string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}
template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto& x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}
template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ")";
}
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}
void debug_out() { cout << endl; }
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cout << " " << to_string(H);
  debug_out(T...);
}
const long long mod = (long long)1000000007;
const long long inf = (long long)2e18;
char nxt(vector<char> v) {
  for (char ch = 'a'; ch <= 'z'; ++ch) {
    bool ok = true;
    for (char tmp : v) ok &= (ch != tmp);
    if (ok) return ch;
  }
  assert(false);
}
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout << fixed << setprecision(9);
  long long n, diff;
  cin >> n >> diff;
  string s1, s2;
  cin >> s1 >> s2;
  long long eq = 0;
  for (long long i = 0; i < n; ++i) eq += (s1[i] == s2[i]);
  long long uneq = n - eq;
  long long same = n - diff;
  long long need_eq = min(same, eq);
  same -= min(same, eq);
  long long need_uneq = same;
  42;
  if (same > uneq / 2) {
    cout << "-1\n";
    return 0;
  }
  string ans(n, '.');
  for (long long i = 0; i < n; ++i) {
    if (need_eq <= 0) break;
    if (s1[i] == s2[i]) {
      ans[i] = s1[i];
      --need_eq;
    }
  }
  long long one = need_uneq;
  long long two = need_uneq;
  for (long long i = 0; i < n; ++i) {
    if (one <= 0) break;
    if (ans[i] == '.') {
      if (s1[i] != s2[i]) {
        ans[i] = s1[i];
        --one;
      }
    }
  }
  for (long long i = 0; i < n; ++i) {
    if (two <= 0) break;
    if (ans[i] == '.') {
      if (s1[i] != s2[i]) {
        ans[i] = s2[i];
        --two;
      }
    }
  }
  for (long long i = 0; i < n; ++i) {
    if (ans[i] == '.') {
      ans[i] = nxt({s1[i], s2[i]});
    }
  }
  cout << ans << '\n';
  return 0;
}
