let inputs = System.Console.ReadLine().Split(' ')
let args = seq{ for str in inputs do yield int(str) }

let matchPair a  b =
    match a,b with
    | a,b   when (a + 1) <=  b -> 0
    | a,b   when (a + 1) >   b -> (a + 1) - b
    | 0,_ -> 0
    | _,0 -> 0
    
printfn "%i" (matchPair (Seq.head args) (Seq.item 1 args))