C++ :

#include<iostream>
using namespace std;
int main()
{
	int m,n=1,sum=0;
	cin>>m;
	do
	{
		sum+=n;
		n++;
	} while (sum<=m);
	cout<<n-1<<endl;
	return 0;
}

Pascal :

var n,a,zf:longint;
begin
  readln(n);
  a:=1;
  zf:=0;
  while zf<n do
             begin
               zf:=zf+a;
               if zf<=n then a:=a+1;
             end;
  writeln(a);
end.