BACTHANG1 pascal

var n:longint;
    f:array[0..55] of int64;

Procedure giai;
var i:longint;
begin
    f[2]:=1;f[3]:=2;
    for i:=4 to n do
        f[i]:=f[i-1]+f[i-2];
    write(f[n]);
end;

begin
    assign(input,'bacthang1.inp'); reset(input);
    assign(output,'bacthang1.out'); rewrite(output);
    read(n);
    giai;
end.