#include <iostream>
#include <cstdio>
#include <algorithm>
#include <vector>
#include <string>
#include <cstring>
#include <sstream>
#include <cmath>
using namespace std;
#define pb push_back
#define f first
#define s second
#define mp make_pair
typedef long long ll;
typedef pair <int, int> pii;
typedef double db;
const int N = 2e5 + 5;
ll f[3][N + 10], ans, n;
string a, b;
int main()
{
  freopen("dictionary.in", "r", stdin);
  freopen("dictionary.out", "w", stdout);
  cin >> n >> a >> b;
  if (a >= b)
  {
    cout << -1;
    return 0;
  }
  int inc = 0;
  while (inc < a.size() && inc < b.size() && a[inc] == b[inc]) inc++;
  a.erase(0, inc);
  b.erase(0, inc);
  f[0][1] = a.size() >= 1;
  f[2][1] = 1;
  if (a.size()) f[1][1] = b[0] - a[0] - 1;
  else f[1][1] = b[0] - 'a';
  for (int i = 1; n > 0 && i < N; i++)
  {

    if (i < a.size())
    {
      f[0][i + 1] += f[0][i];
      f[1][i + 1] += f[0][i] * ('z' - a[i]);
    } else f[1][i + 1] += f[0][i] * 26;

    f[1][i + 1] += f[1][i] * 26;


    if (i < b.size())
    {
      f[2][i + 1] += f[2][i];
      f[2][i + 1] += f[2][i] * (b[i] - 'a');
    }

    ll sp = f[1][i] + f[2][i] * (i < b.size());
    ans += min(n, sp) * (inc + i);
    n -= sp;
  }
  if (n > 0)
  {
    cout << -1;
    return 0;
  }
  cout << ans;
/*
2
a
b
4

2
ab
ba
3

3
ba
ab
-1

5
abcdabcd
abcdabcz


1
aa
aaaa

670
ab
ba
1984

27
abcd
abcf
134
*/
  return 0;
}