Language: C++

using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Text;
namespace AtCoder {
    class MainClass {
        //C#7.0
        int X;
        void Solve() {
            io.i(out X);

            Mathf mat = new Mathf();

            int ans;
            for (ans = 1; ;ans++)
                if (mat.TousaSum(1, 1, ans) >= X)
                    break;

            io.o(ans);

        }

        public static void Main(string[] args) { new MainClass().Stream(); }
        IO io = new IO();
        void Stream() { Solve(); io.writeFlush(); }

        class IO {
            string[] nextBuffer; int BufferCnt; char[] cs = new char[] { ' ' };
            StreamWriter sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
            public IO() { nextBuffer = new string[0]; BufferCnt = 0; Console.SetOut(sw); }
            public string next() {
                if (BufferCnt < nextBuffer.Length) return nextBuffer[BufferCnt++];
                string st = Console.ReadLine();
                while (st == "") st = Console.ReadLine();
                nextBuffer = st.Split(cs, StringSplitOptions.RemoveEmptyEntries);
                BufferCnt = 0;
                return nextBuffer[BufferCnt++];
            }
            public char nextChar() { return char.Parse(next()); }
            public int nextInt() { return int.Parse(next()); }
            public long nextLong() { return long.Parse(next()); }
            public double nextDouble() { return double.Parse(next()); }
            public string[] array() { return Console.ReadLine().Split(' '); }
            public char[] arrayChar() { return Array.ConvertAll(array(), char.Parse); }
            public int[] arrayInt() { return Array.ConvertAll(array(), int.Parse); }
            public long[] arrayLong() { return Array.ConvertAll(array(), long.Parse); }
            public double[] arrayDouble() { return Array.ConvertAll(array(), double.Parse); }
            private bool typeEQ<T, U>() { return typeof(T).Equals(typeof(U)); }
            private T convertType<T, U>(U v) { return (T)Convert.ChangeType(v, typeof(T)); }
            private T suitType<T>(string s) {
                if (typeEQ<T, int>()) return convertType<T, int>(int.Parse(s));
                if (typeEQ<T, long>()) return convertType<T, long>(long.Parse(s));
                if (typeEQ<T, double>()) return convertType<T, double>(double.Parse(s));
                if (typeEQ<T, char>()) return convertType<T, char>(char.Parse(s));
                return convertType<T, string>(s);
            }
            public void i<T>(out T v) { v = suitType<T>(next()); }
            public void i<T, U>(out T v1, out U v2) {
                var a = array(); v1 = suitType<T>(a[0]); v2 = suitType<U>(a[1]);
            }
            public void i<T, U, V>(out T v1, out U v2, out V v3) {
                var a = array(); v1 = suitType<T>(a[0]); v2 = suitType<U>(a[1]);
                v3 = suitType<V>(a[2]);
            }
            public void i<T, U, V, W>(out T v1, out U v2, out V v3, out W v4) {
                var a = array(); v1 = suitType<T>(a[0]); v2 = suitType<U>(a[1]);
                v3 = suitType<V>(a[2]); v4 = suitType<W>(a[3]);
            }
            public void ini(out int[] a, int size) {
                a = new int[size]; for (int i = 0; i < size; i++) a[i] = nextInt();
            }
            public void ini(out int[] a1, out int[] a2, int size) {
                a1 = new int[size]; a2 = new int[size];
                for (int i = 0; i < size; i++) { a1[i] = nextInt(); a2[i] = nextInt(); }
            }
            public void ini(out string[] a, int size) {
                a = new string[size]; for (int i = 0; i < size; i++) a[i] = next();
            }
            public void inii(out int[] a, int size) {
                a = new int[size]; for (int i = 0; i < size; i++) a[i] = nextInt() - 1;
            }
            public void inii(out int[] a1, out int[] a2, int size) {
                a1 = new int[size]; a2 = new int[size];
                for (int i = 0; i < size; i++) { a1[i] = nextInt() - 1; a2[i] = nextInt() - 1; }
            }
            public void o<T>(T v) { Console.WriteLine(v); }
            public void o<T>(params T[] a) { foreach (T v in a) Console.WriteLine(v); }
            public void o<T>(List<T> l) { foreach (T v in l) sw.WriteLine(v); }
            public void ol<T>(T v) { Console.Write(v + " "); }
            public void ol<T>(params T[] a) { foreach (T v in a) Console.Write(v + " "); Console.WriteLine(""); }
            public void YN(bool f) { Console.WriteLine(f ? "YES" : "NO"); }
            public void Yn(bool f) { Console.WriteLine(f ? "Yes" : "No"); }
            public void yn(bool f) { Console.WriteLine(f ? "yes" : "no"); }
            public void br(){Console.WriteLine("");}
            public void writeFlush() { Console.Out.Flush(); }
            public string pad0<T>(T v, int n) { return v.ToString().PadLeft(n, '0'); }
        }

        class Mathf {
            public int mod = 1000000007;//10^9+7
            public long Pow(long a, long b) {
                if (b == 0) return 1;
                if (b % 2 == 1) return (a % mod * Pow(a % mod, b - 1) % mod) % mod;
                else return Pow(a * a % mod, b / 2) % mod;
            }
            public long Fact(long n) { return n != 0 ? (n % mod * (Fact(n - 1) % mod) % mod) : 1; }
            public long C(long n, long r) {
                if (r == 0 || n == r) return 1;
                else return (Fact(n) % mod * Pow((Fact(n - r) % mod * Fact(r) % mod) % mod, mod - 2) % mod) % mod;
            }
            public long Sum(params long[] a) { return a.Aggregate((v, next) => (v % mod + next) % mod); }
            public int GCD(int a, int b) { if (a < b) Swap(ref a, ref b); return b == 0 ? a : GCD(b, a % b); }
            public int GCD(int[] array) { return array.Aggregate((v, next) => GCD(v, next)); }
            public void Swap<T>(ref T a, ref T b) { T tmp = a; a = b; b = tmp; }
            public T Max<T>(params T[] v) { return v.Max(); }
            public T Min<T>(params T[] v) { return v.Min(); }
            public double Dis(int x1, int y1, int x2, int y2) {
                return Math.Sqrt(Math.Pow((x2 - x1), 2) + Math.Pow((y2 - y1), 2));
            }
            public int Digit(long n) { return (n == 0) ? 1 : (int)Math.Log10(n) + 1; }
            public int Clamp(int n, int a, int b) {return (n < a) ? a : (b < n) ? b : n;}
            public long Clamp(long n, long a, long b) {return (n < a) ? a : (b < n) ? b : n;}
            public double Clamp(double n, double a, double b) { return (n < a) ? a : (b < n) ? b : n;}
            public long Tousa(long a, long d, int no) { return a + (no - 1) * d; }
            public long TousaSum(long a, long d, int no) { return no* (2 * a + (no - 1) * d) / 2; }
        }
    }
    class AssociativeArray<T>{
        public Dictionary<T, int> dic;
        public void init(T[] a){dic = new Dictionary<T, int> ();
            for (int i = 0;i<a.Length;i++){if (!dic.ContainsKey(a[i])) dic[a[i]] = 0;dic[a[i]]++;}
        }
        public int ValueSum() { int s = 0; foreach (var i in dic) s += i.Value; return s; }
        public int KeyNum() { return dic.Keys.Count; }
        public int Max(){return dic.Values.Max();}public int Min() { return dic.Values.Min(); }
        public T MaxKey() { return dic.First(d => d.Value == Max()).Key; }
        public T MinKey() { return dic.First(d => d.Value == Min()).Key; }
        public bool isContains(T k) { return dic.ContainsKey(k); }
        //:sort->array
    }
    class Time{
        public bool isLeapYear(int y) { return (y % 400 == 0 || (y % 4 == 0 && y % 100 != 0)); }
    }
    class BucketSort {
        public int[] bucket;
        public void Init(int[] a) {
            bucket = new int[a.Max() + 1];
            for (int i = 0; i < a.Length; ++i) ++bucket[a[i]];
            for (int i = 0, j = 0; j < bucket.Length; ++j)
                for (int k = bucket[j]; k != 0; --k, ++i) a[i] = j;
        }
    }

    static class StringEX {
        public static string Reversed(this string s) { return string.Join("", s.Reverse()); }
        public static string Repeat(this string s, int n) { return string.Concat(Enumerable.Repeat(s, n).ToArray()); }
    }
    static class CharEX{
        public static int toInt(this char c) { return int.Parse(c.ToString()); }
    }
}

Compiler output:
a.cc:1:7: error: expected nested-name-specifier before 'System'
    1 | using System;
      |       ^~~~~~
a.cc:2:7: error: expected nested-name-specifier before 'System'
    2 | using System.Collections;
      |       ^~~~~~
a.cc:3:7: error: expected nested-name-specifier before 'System'
    3 | using System.Collections.Generic;
      |       ^~~~~~
a.cc:4:7: error: expected nested-name-specifier before 'System'
    4 | using System.Diagnostics;
      |       ^~~~~~
a.cc:5:7: error: expected nested-name-specifier before 'System'
    5 | using System.IO;
      |       ^~~~~~
a.cc:6:7: error: expected nested-name-specifier before 'System'
    6 | using System.Linq;
      |       ^~~~~~
a.cc:7:7: error: expected nested-name-specifier before 'System'
    7 | using System.Text;
      |       ^~~~~~
a.cc:26:15: error: expected ':' before 'static'
   26 |         public static void Main(string[] args) { new MainClass().Stream(); }
      |               ^~~~~~~
      |               :
a.cc:26:33: error: 'string' has not been declared
   26 |         public static void Main(string[] args) { new MainClass().Stream(); }
      |                                 ^~~~~~
a.cc:26:42: error: expected ',' or '...' before 'args'
   26 |         public static void Main(string[] args) { new MainClass().Stream(); }
      |                                          ^~~~
a.cc:27:9: error: 'IO' does not name a type
   27 |         IO io = new IO();
      |         ^~
a.cc:31:13: error: 'string' does not name a type
   31 |             string[] nextBuffer; int BufferCnt; char[] cs = new char[] { ' ' };
      |             ^~~~~~
a.cc:31:53: error: expected unqualified-id before '[' token
   31 |             string[] nextBuffer; int BufferCnt; char[] cs = new char[] { ' ' };
      |                                                     ^
a.cc:32:13: error: 'StreamWriter' does not name a type
   32 |             StreamWriter sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
      |             ^~~~~~~~~~~~
a.cc:33:19: error: expected ':' before 'IO'
   33 |             public IO() { nextBuffer = new string[0]; BufferCnt = 0; Console.SetOut(sw); }
      |                   ^~~
      |                   :
a.cc:34:19: error: expected ':' before 'string'
   34 |             public string next() {
      |                   ^~~~~~~
      |                   :
a.cc:34:20: error: 'string' does not name a type
   34 |             public string next() {
      |                    ^~~~~~
a.cc:42:19: error: expected ':' before 'char'
   42 |             public char nextChar() { return char.Parse(next()); }
      |                   ^~~~~
      |                   :
a.cc:43:19: error: expected ':' before 'int'
   43 |             public int nextInt() { return int.Parse(next()); }
      |                   ^~~~
      |                   :
a.cc:44:19: error: expected ':' before 'long'
   44 |             public long nextLong() { return long.Parse(next()); }
      |                   ^~~~~
      |                   :
a.cc:45:19: error: expected ':' before 'double'
   45 |             public double nextDouble() { return double.Parse(next()); }
      |                   ^~~~~~~
      |                   :
a.cc:46:19: error: expected ':' before 'string'
   46 |             public string[] array() { return Console.ReadLine().Split(' '); }
      |                   ^~~~~~~
      |                   :
a.cc:46:20: error: 'string' does not name a type
   46 |             public string[] array() { return Console.ReadLine().Split(' '); }
      |                    ^~~~~~
a.cc:47:19: error: expected ':' before 'char'
   47 |             public char[] arrayChar() { return Array.ConvertAll(array(), char.Parse); }
      |                   ^~~~~
      |                   :
a.cc:47:24: error: expected unqualified-id before '[' token
   47 |             public char[] arrayChar() { return Array.ConvertAll(array(), char.Parse); }
      |                        ^
a.cc:48:19: error: expected ':' before 'int'
   48 |             public int[] arrayInt() { return Array.ConvertAll(array(), int.Parse); }
      |                   ^~~~
      |                   :
a.cc:48:23: error: expected unqualified-id before '[' token
   48 |             public int[] arrayInt() { return Array.ConvertAll(array(), int.Parse); }
      |                       ^
a.cc:49:19: error: expected ':' before 'long'
   49 |             public long[] arrayLong() { return Array.ConvertAll(array(), long.Parse); }
      |                   ^~~~~
      |                   :
a.cc:49:24: error: expected unqualified-id before '[' token
   49 |             public long[] arrayLong() { return Array.ConvertAll(array(), long.Parse); }
      |                        ^
a.cc:50:19: error: expected ':' before 'double'
   50 |             public double[] arrayDouble() { return Array.ConvertAll(array(), double.Parse); }
      |                   ^~~~~~~
      |                   :
a.cc:50:26: error: expected unqualified-id before '[' token
   50 |             public double[] arrayDouble() { return Array.ConvertAll(array(), double.Parse); }
      |                          ^
a.cc:51:20: error: expected ':' before 'bool'
   51 |             private bool typeEQ<T, U>() { return typeof(T).Equals(typeof(U)); }
      |                    ^~~~~
      |                    :
a.cc:51:26: error: expected ';' at end of member declaration
   51 |             private bool typeEQ<T, U>() { return typeof(T).Equals(typeof(U)); }
      |                          ^~~~~~
      |                                ;
a.cc:51:32: error: expected unqualified-id before '<' token
   51 |             private bool typeEQ<T, U>() { return typeof(T).Equals(typeof(U)); }
      |                                ^
a.cc:52:20: error: expected ':' before 'T'
   52 |             private T convertType<T, U>(U v) { return (T)Convert.ChangeType(v, typeof(T)); }
      |                    ^~
      |                    :
a.cc:52:21: error: 'T' does not name a type
   52 |             private T convertType<T, U>(U v) { return (T)Convert.ChangeType(v, typeof(T)); }
      |                     ^
a.cc:53:20: error: expected ':' before 'T'
   53 |             private T suitType<T>(string s) {
      |                    ^~
      |                    :
a.cc:53:21: error: 'T' does not name a type
   53 |             private T suitType<T>(string s) {
      |                     ^
a.cc:60:19: error: expected ':' before 'void'
   60 |             public void i<T>(out T v) { v = suitType<T>(next()); }
      |                   ^~~~~
      |                   :
a.cc:60:25: error: variable or field 'i' declared void
   60 |             public void i<T>(out T v) { v = suitType<T>(next()); }
      |                         ^
a.cc:60:25: error: expected ';' at end of member declaration
   60 |             public void i<T>(out T v) { v = suitType<T>(next()); }
      |                         ^
      |                          ;
a.cc:60:26: error: expected unqualified-id before '<' token
   60 |             public void i<T>(out T v) { v = suitType<T>(next()); }
      |                          ^
a.cc:61:19: error: expected ':' before 'void'
   61 |             public void i<T, U>(out T v1, out U v2) {
      |                   ^~~~~
      |                   :
a.cc:61:25: error: variable or field 'i' declared void
   61 |             public void i<T, U>(out T v1, out U v2) {
      |                         ^
a.cc:61:25: error: expected ';' at end of member declaration
   61 |             public void i<T, U>(out T v1, out U v2) {
      |                         ^
      |                          ;
a.cc:61:26: error: expected unqualified-id before '<' token
   61 |             public void i<T, U>(out T v1, out U v2) {
      |                          ^
a.cc:64:19: error: expected ':' before 'void'
   64 |             public void i<T, U, V>(out T v1, out U v2, out V v3) {
      |                   ^~~~~
      |                   :
a.cc:64:25: error: variable or field 'i' declared void
   64 |             public void i<T, U, V>(out T v1, out U v2, out V v3) {
      |                         ^
a.cc:64:25: error: expected ';' at end of member declaration
   64 |             public void i<T, U, V>(out T v1, out U v2, out V v3) {
      |                         ^
      |                          ;
a.cc:64:26: error: expected unqualified-id before '<' token
   64 |             public void i<T, U, V>(out T v1, out U v2, out V v3) {
      |                          ^
a.cc:68:19: error: expected ':' before 'void'
   68 |             public void i<T, U, V, W>(out T v1, out U v2, out V v3, out W v4) {
      |                   ^~~~~
      |                   :
a.cc:68:25: error: variable or field 'i' declared void
   68 |             public void i<T, U, V, W>(out T v1, out U v2, out V v3, out W v4) {
      |                         ^
a.cc:68:25: error: expected ';' at end of member declaration
   68 |             public void i<T, U, V, W>(out T v1, out U v2, out V v3, out W v4) {
      |                         ^
      |                          ;
a.cc:68:26: error: expected unqualified-id before '<' token
   68 |             public void i<T, U, V, W>(out T v1, out U v2, out V v3, out W v4) {
      |                          ^
a.cc:72:19: error: expected ':' before 'void'
   72 |             public void ini(out int[] a, int size) {
      |                   ^~~~~
      |                   :
a.cc:72:29: error: 'out' has not been declared
   72 |             public void ini(out int[] a, int size) {
      |                             ^~~
a.cc:72:29: error: two or more data types in declaration of 'parameter'
a.cc:72:38: error: expected ')' before 'a'
   72 |             public void ini(out int[] a, int size) {
      |                            ~         ^~
      |                                      )
a.cc:72:37: error: expected ';' at end of member declaration
   72 |             public void ini(out int[] a, int size) {
      |                                     ^
      |                                      ;
a.cc:72:39: error: 'a' does not name a type
   72 |             public void ini(out int[] a, int size) {