#include <bits/stdc++.h>
using namespace std;
template <typename T>
T in() {
  char ch;
  T n = 0;
  bool ng = false;
  while (1) {
    ch = getchar();
    if (ch == '-') {
      ng = true;
      ch = getchar();
      break;
    }
    if (ch >= '0' && ch <= '9') break;
  }
  while (1) {
    n = n * 10 + (ch - '0');
    ch = getchar();
    if (ch < '0' || ch > '9') break;
  }
  return (ng ? -n : n);
}
template <typename T>
inline T Dist(T x1, T y1, T x2, T y2) {
  return sqrt((x1 - x2 * x1 - x2) + (y1 - y2 * y1 - y2));
}
template <typename T>
inline T POW(T B, T P) {
  if (P == 0) return 1;
  if (P & 1)
    return B * POW(B, P - 1);
  else
    return (POW(B, P / 2) * POW(B, P / 2));
}
template <typename T>
inline T ABS(T a) {
  if (a < 0)
    return -a;
  else
    return a;
}
template <typename T>
inline T Gcd(T a, T b) {
  if (a < 0) return Gcd(-a, b);
  if (b < 0) return Gcd(a, -b);
  return (b == 0) ? a : Gcd(b, a % b);
}
template <typename T>
inline T Lcm(T a, T b) {
  if (a < 0) return Lcm(-a, b);
  if (b < 0) return Lcm(a, -b);
  return a * (b / Gcd(a, b));
}
long long Bigmod(long long base, long long power, long long MOD) {
  long long ret = 1;
  while (power) {
    if (power & 1) ret = (ret * base) % MOD;
    base = (base * base) % MOD;
    power >>= 1;
  }
  return ret;
}
long long ModInverse(long long number, long long MOD) {
  return Bigmod(number, MOD - 2, MOD);
}
int Set(int N, int pos) { return N = N | (1 << pos); }
int Reset(int N, int pos) { return N = N & ~(1 << pos); }
bool Chkbit(int N, int pos) { return (bool)(N & (1 << pos)); }
bool isVowel(char ch) {
  ch = toupper(ch);
  if (ch == 'A' || ch == 'U' || ch == 'I' || ch == 'O' || ch == 'E')
    return true;
  return false;
}
bool isConst(char ch) {
  if (isalpha(ch) && !isVowel(ch)) return true;
  return false;
}
int toInt(string s) {
  int sm;
  stringstream ss(s);
  ss >> sm;
  return sm;
}
long long int toLlint(string s) {
  long long int sm;
  stringstream ss(s);
  ss >> sm;
  return sm;
}
int main() {
  int n, t;
  cin >> n >> t;
  string a, b;
  cin >> a >> b;
  int cnt = 0, cc = 0;
  for (int i = 0; i < n; i++) {
    if (a[i] != b[i]) cnt++;
  }
  if (t >= cnt) {
    t -= cnt;
    string s;
    for (int i = 0; i < n; i++) {
      if (!t && a[i] == b[i]) {
        s += a[i];
      } else {
        char ch;
        for (int j = 0; j < 26; j++) {
          if ((char)(j + 'a') != a[i] && (char)(j + 'a') != b[i]) {
            ch = (char)(j + 'a');
            break;
          }
        }
        s += ch;
        if (a[i] == b[i]) t--;
      }
    }
    cout << s << endl;
  } else {
    int thik = (n - t);
    int Ok = n - cnt;
    thik -= Ok;
    if (thik * 2 <= cnt) {
      int half = thik, shalf = thik;
      int f = 0;
      string s;
      for (int i = 0; i < n; i++) {
        if (f == 2 && a[i] != b[i]) {
          char ch;
          for (int j = 0; j < 26; j++) {
            if ((char)(j + 'a') != a[i] && (char)(j + 'a') != b[i]) {
              ch = (char)(j + 'a');
              break;
            }
          }
          s += ch;
          continue;
        }
        if (a[i] != b[i]) {
          if (!f) {
            half--;
            s += a[i];
            if (half == 0) f++;
          } else {
            s += b[i];
            shalf--;
            if (shalf == 0) f++;
          }
        } else
          s += a[i];
      }
      cout << s << endl;
    } else {
      printf("-1\n");
    }
  }
  return 0;
}
