負の数を書く時は、-と1の間にスペース入れる癖つけといた方がよさげ

> let x = (3,4);;
val x : int * int = (3, 4)

> fst x;;
val it : int = 3
> fst x - 1;;
val it : int = 2
> fst x -1;;

  fst x -1;;
  ----^

stdin(15,5): error FS0001: Type mismatch. Expecting a
    ('a -> 'b) * 'c    
but given a
    int * int    
The type ''a -> 'b' does not match the type 'int'
> (fst x) -1;;

  (fst x) -1;;
  -^^^^^

stdin(16,2): error FS0003: This value is not a function and cannot be applied
> (fst x) - 1;;
val it : int = 2