#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, t;
  string a, b;
  cin >> n >> t;
  cin >> a >> b;
  int eq = 0;
  string ans = "";
  for (int i = 0; i < (n); i++) {
    if (a[i] == b[i]) {
      eq++;
      if (eq <= (n - t))
        ans += a[i];
      else
        ans += "*";
    } else
      ans += "*";
  }
  int eq1 = eq;
  int eq2 = eq;
  if (eq >= (n - t)) {
    for (int i = 0; i < (n); i++) {
      if (ans[i] == '*') {
        for (int j = 0; j < (26); j++) {
          char c = (char)('a' + j);
          if (c != a[i] and c != b[i]) {
            ans[i] = c;
            break;
          }
        }
      }
    }
  } else {
    for (int i = 0; i < (n); i++) {
      if (ans[i] != '*') continue;
      if (eq1 <= eq2) {
        if (eq1 < (n - t)) {
          ans[i] = a[i];
          eq1++;
        }
      } else {
        ans[i] = b[i];
        eq2++;
      }
      if (ans[i] == '*') {
        for (int j = 0; j < (26); j++) {
          char c = (char)('a' + j);
          if (c != a[i] and c != b[i]) {
            ans[i] = c;
            break;
          }
        }
      }
    }
  }
  int dif1 = 0;
  int dif2 = 0;
  for (int i = 0; i < (n); i++) {
    if (ans[i] != a[i]) dif1++;
    if (ans[i] != b[i]) dif2++;
  }
  if (dif1 == t and dif2 == t)
    cout << ans << endl;
  else
    cout << -1 << endl;
  return 0;
}
