Given a permutation $p$ of length $n$, we define the following functions

$S(p) =$ { $i : p(i) > p(i+1), 1 \le i \le n-1$ }

$f(p)$ = Sum of elements of $S(p)$.

Let $F(n,k)$ = number of permutations $p$ of length $n$ with $f(p) = k$.

find $F(123,321)$ modulo $10^9 +7$