Let $R$ be Dedekind domain and $I$ be $R$'s fractional ideal. Then my book (Advanced topics in the arithmetic of elliptic curves, written by Silvermn, p113) reads there exists resolution(exact sequence)
$R_K^m→R_K^n→I→0$ (The first arrow is producing $m×n$ matrices with coeffieints in $R_K$).
But for given $I$, how we take $m,n$ and why is this an exact sequence ?