This is my program:

{$N+}
var
  n,m:extended;

begin
  read(m);
  n:=(sqrt(8*m+1)-1)/2;
  writeln(n:0:0);
end.

Could you help me??

Oh God!
Do you really think that extended holds 600 digits?
You should implement that formula on big numbers...

But how to calculate "sqrt"? Could you help me? Thank you!

Hi!

Here is a link to explanation of one very interesting and simple sqrt algorithm http://www.homeschoolmath.net/other_topics/square-root-algorithm-example.php
I got AC using it, so if you have any questions about the implementation email me: dporobic@eunet.yu

Still WA!!! Why?

You implement sqrt for big nums? if so, post your source. I'll      try to debug it...

This is my program:

{$N+}
var
  flag:boolean;
  s:string;
  a,b:array [1..601] of longint;
  x,temp,last,lasttemp:extended;
  l,k:longint;
  code,i,j:integer;

procedure pingfanggen;
begin
  if odd(l) then
    begin
      inc(l);
      a[l]:=0;
    end;
  for i:=9 downto 1 do
    if i*i<=a[l]*10+a[l-1] then
      begin
        last:=a[l]*10+a[l-1]-i*i;
        b[1]:=i;
        k:=1;
        break;
      end;
  lasttemp:=0;
  for i:=(l-1) div 2 downto 1 do
    begin
      x:=a[i*2]*10+a[i*2-1]+last*100;
      inc(k);
      temp:=((b[k-1]*2)+lasttemp)*10;
      for j:=9 downto 0 do
        if (temp+j)*j<=x then
          begin
            temp:=temp+j;
            break;
          end;
      b[k]:=j;
      last:=x-b[k]*temp;
      lasttemp:=temp;
    end;
end;

begin
  readln(s);
  l:=length(s);
  for i:=l downto 1 do
    val(s[l-i+1],a[i],code);
  for i:=1 to l do
    a[i]:=a[i]*8;
  a[1]:=a[1]+1;
  for i:=1 to l+2 do
    if a[i]>9 then
      begin
        a[i+1]:=a[i+1]+a[i] div 10;
        a[i]:=a[i] mod 10;
        if i+1>l then l:=i+1;
      end;
  pingfanggen;
  fillchar(a,sizeof(a),0);
  for i:=1 to k do
    a[i]:=b[k-i+1];
  l:=k;
  a[1]:=a[1]-1;
  for i:=1 to l do
    if a[i]<0 then
      begin
        a[i+1]:=a[i+1]-1;
        a[i]:=a[i]+10;
      end;
  for i:=l downto 1 do
    begin
      a[i-1]:=a[i-1]+10*(a[i] mod 2);
      a[i]:=a[i] div 2;
    end;
  flag:=true;
  for i:=l downto 1 do
    if (flag=false)or(a[i]<>0) then begin write(a[i]); flag:=false; end;
  writeln;
end.

Could you help me? Thank you!

I debugged your program and got different answers for almost all tests I've tryied... For example: 1543209876540123456790
 -> 55555555555 (correct), yours -> 55531491125 and many more...
I really don't have any idea how did your program passed 5 tests...

thanks

long alternative trunc(sqrt(2 * n))
sorry for my english)