#pragma GCC optimize("O3") #pragma GCC optimize ("unroll-loops") // #pragma GCC target("avx2") #include <bits/stdc++.h> #define ll long long #define ull unsigned ll #define ld long double #define all(v, l) v.begin() + l, v.end() #define rall(v, l) v.rbegin(), v.rend() - l #define pb push_back #define pf push_front #define rsz resize #define fi first #define se second #define LMAX LLONG_MAX #define LMIN LLONG_MIN #define IMAX INT_MAX #define IMIN INT_MIN #define endl "\n" #define newline cout << endl; using namespace std; const ll MOD = 1000000007; // structs // globals // variables int n, s, e; // iterators int i, j; // notes /* -stuff you should look for- * int overflow, array bounds * special cases (n=1?) * do something instead of nothing and stay organized * WRITE STUFF DOWN * DON'T GET STUCK ON ONE APPROACH continue - skip the rest in the loop */ // functions ll GCD(ll numeroune, ll numerodeux); ll LCM(ll numeroune, ll numerodeux); ll power(ll numeroune, ll numerodeux); void solve() { cin >> n >> s >> e; vector <vector <ll> > dp(n + 1, vector <ll>(n + 2)); dp[1][1] = 1LL; for (i = 1; i <= n; i++) { for (j = 1; j <= i; j++) { if (i == s || i == e) { dp[i][j + 1] = (dp[i][j + 1] + dp[i - 1][j]) % MOD; dp[i][j] = (dp[i][j] + dp[i - 1][j]) % MOD; } else { dp[i][j + 1] = (dp[i][j + 1] + ((dp[i - 1][j] * 1LL * (j + 1 - (i >= s) - (i >= e))) % MOD)) % MOD; dp[i][j - 1] = (dp[i][j - 1] + ((dp[i - 1][j] * 1LL * (j - 1)) % MOD)); } } } cout << dp[n][1]; } int main() { ios_base::sync_with_stdio(0); cin.tie(0); int t = 1; // cin >> t; while (t--) { solve(); newline } } ll GCD(ll numeroune, ll numerodeux) { if (!numeroune) { return numerodeux; } return GCD(numerodeux % numeroune, numeroune); } ll LCM(ll numeroune, ll numerodeux) { return numeroune * numerodeux / GCD(numeroune, numerodeux); } ll power(ll numeroune, ll numerodeux) { ll res = 1; while (numerodeux) { if (numerodeux & 1) { res *= numeroune; } numeroune *= numeroune; numerodeux >>= 1; } return res; } /* $$$$$$$$\ $$$$$$$$\ $$ _____|\____$$ | $$ | $$ / $$$$$\ $$ / $$ __| $$ / $$ | $$ / $$$$$$$$\ $$$$$$$$\ \________|\________| */