DP の基本きほんパターン

#entry main
#indent 4
#target std

#import "core/math" as *
#import "core/result" as *
#import "core/cast" as *
#import "std/streamio" as *
#import "std/iotarget" as *

fn ways <(i32)*>i64> (n):
    if le n 1:
        then <i64> cast 1
        else:
            let mut a <i64> cast 1;
            let mut b <i64> cast 1;
            let mut i <i32> 2;
            while le i n:
                do:
                    let c <i64> add a b;
                    set a b;
                    set b c;
                    set i add i 1;
            b

fn main <()*> ()> ():
    let sc <StreamScanner> unwrap_ok open ReadStream::Stdio;
    let ans <i64> ways read sc;
    close sc;
    unwrap_ok open WriteStream::Stdio
    |> writeln ans
    |> flush
    |> close