#include <bits/stdc++.h> #include <ext/pb_ds/assoc_container.hpp> #include <ext/pb_ds/tree_policy.hpp> using namespace __gnu_pbds; using namespace std; #ifdef LOCAL #include "debug.h" #else #define debug(...) 42 #endif #define ll long long #define int ll #define ld long double #define pii pair<int,int> #define all(x) (x).begin(), (x).end() #define isz(x) (int)(x.size()) #define vi vector<int> #define vvi vector<vi> #define ost tree<pii, null_type, less<pii>, rb_tree_tag, tree_order_statistics_node_update> mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count()); const int sz = 2200, inf = 1000000000; const ll mod = 1000000007, INF = 1000000000000000000; int dp[sz][sz]; void solve(int tc) { int n, cs, cf; cin >> n >> cs >> cf; dp[1][1] = 1; int cnt = 0; for (int i = 1; i <= n; i++) { if (i == cs || i == cf) cnt++; for (int j = 1; j < i; j++) { if (i == cs || i == cf) { dp[i][j+1] += dp[i-1][j]; dp[i][j] += dp[i-1][j]; dp[i][j+1] %= mod; dp[i][j] %= mod; } else { dp[i][j+1] += (j + 1 - cnt) * dp[i-1][j]; dp[i][j-1] += (j - 1) * dp[i-1][j]; dp[i][j+1] %= mod; dp[i][j-1] %= mod; } } } cout << dp[n][1] << '\n'; } void precompute() {} signed main() { ios::sync_with_stdio(false); cin.tie(nullptr); #ifdef LOCAL freopen("err.log", "w", stderr); freopen("in.txt", "r", stdin); #endif precompute(); int t = 1; // cin >> t; for (int tc = 1; tc <= t; tc++) solve(tc); return 0; }