/**
*    author:  tomo0608
*    created: 27.02.2021 21:15:09
**/

#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef long double ld;
template <class T> using V = vector<T>;
template <class T> using VV = V<V<T>>;
typedef pair<int,int> pii;
typedef pair<long long, long long> pll;

#define all(x) x.begin(),x.end()
#define rep2(i, m, n) for (int i = (m); i < (n); ++i)
#define rep(i, n) rep2(i, 0, n)
#define drep2(i, m, n) for (int i = (m)-1; i >= (n); --i)
#define drep(i, n) drep2(i, n, 0)
#define unique(a) a.erase(unique(a.begin(),a.end()),a.end())
template<class T> using priority_queue_rev = priority_queue<T, vector<T>, greater<T> >;
template<class T, class U> inline bool chmax(T &a, const U &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T, class U> inline bool chmin(T &a, const U &b) { if (a>b) { a=b; return 1; } return 0; }
template<class T1, class T2> istream &operator>>(istream &is, pair<T1, T2> &p) { is >> p.first >> p.second; return is; }
template<class T1, class T2> ostream &operator<<(ostream &os, const pair<T1, T2> &p) { os << '(' << p.first << ", " << p.second << ')'; return os; }
template<class T> istream &operator>>(istream &is, vector<T> &v) { for (auto &e : v) is >> e; return is; }
template<class T> ostream &operator<<(ostream &os, const vector<T> &v) { for (auto &e : v) os << e << ' '; return os; }
template<typename T> ostream& operator << (ostream& os, set<T>& set_var) {os << "{"; for (auto itr = set_var.begin(); itr != set_var.end(); itr++) {os << *itr;++itr;if(itr != set_var.end()) os << ", ";itr--;}os << "}";return os;}
template <typename T, typename U> ostream &operator<<(ostream &os, map<T, U> &map_var) {os << "{";for(auto itr = map_var.begin(); itr != map_var.end(); itr++) {os << *itr;itr++;if (itr != map_var.end()) os << ", ";itr--;}os << "}";return os;}
template<class T> inline int count_between(vector<T> &a, T l, T r) { return lower_bound(all(a), r) - lower_bound(all(a), l); } // [l, r)
#define pb push_back
#define eb emplace_back
#define elif else if
#define mp make_pair
#define bit(n, k) ((n >> k) & 1) /*nのk bit目*/
template<typename T> T gcd(T x, T y){if(x%y == 0)return y;return gcd(y, x%y);}
template<typename T> T gcd(vector<T> a){T res = a[0];for(auto &x: a)res = gcd(res, x);return res;}
template <typename T>T mypow(T x, ll n){T ret = 1;while(n > 0){if(n & 1)(ret *= x);(x *= x);n >>= 1;}return ret;}
#define endl '\n'
int dx[8] = {1, 0, -1, 0, 1, 1, -1, -1};
int dy[8] = {0, 1, 0, -1, 1, -1, -1, 1};


void solve(){
    ll K;cin >> K;
    string s,t;cin >> s >> t;
    V<int> cards(10, 0), cardt(10, 0);
    rep(i,4){
        cards[s[i] - '0']++;
        cardt[t[i] - '0']++;
    }
    ld ans = 0;
    int sum = 0;
    rep2(i, 1, 10)rep2(j, 1, 10){
        cards[i]++;
        cardt[j]++;
        ll now_s = 0, now_t = 0;
        bool check = true;
        rep(k,10){
            now_s += k * mypow(10, cards[k]);
            now_t += k * mypow(10, cardt[k]);
            if(K - cards[k] - cardt[k] < 0)check = false;
        }
        if(check)sum++;
        cards[i]--;
        cardt[j]--;
        if(check && now_s > now_t){
            //cout << i << ' ' << j << endl;
            ans += ((ld)(K - cards[i] - cardt[i]) * (K - cards[j] - cardt[j] - (i == j)))/ ((9*K - 8) * (9*K - 9));
        }
    }
    cout << ans << endl;
}

int main() {
    cin.tie(0);
    ios_base::sync_with_stdio(false);
    cout << setprecision(20);
    int codeforces = 1;
    //cin >> codeforces;
    while(codeforces--){
        solve();
    }
    return 0;
}
#include <bits/stdc++.h>
using ll = long long;
#define rep(i,n) for(int i = 0;i < (int)(n); i++)
using namespace std;

int main() {
	int A, B, C, D;
	cin >> A >> B >> C >> D;
	if (C * D - B == 0)cout << -1;
	if (C * D - B < 0)cout << -1;
	if (C * D - B > 0){
		int N = A / (C * D - B);
		if (A % (C * D - B) != 0)cout << N + 1;
		if (A % (C * D - B) == 0)cout << N;
	}
}
