Problem:
Score : 300 points

Problem Statement

Given is a positive integer N.
How many tuples (A,B,C) of positive integers satisfy A \times B + C = N?

Constraints

2 \leq N \leq 10^6

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N

Output

Print the answer.

Sample Input 1

3

Sample Output 1

3

There are 3 tuples of integers that satisfy A \times B + C = 3: (A, B, C) = (1, 1, 2), (1, 2, 1), (2, 1, 1).

Sample Input 2

100

Sample Output 2

473

Sample Input 3

1000000

Sample Output 3

13969985

Solution in Go (1.14.1) (judge verdict: Accepted)

package main

import (
  "bufio"
  "os"
  "strconv"
  "strings"
  "fmt"
)

func Solve() {
  var N int
  NextInt(&N)
  N--
  cnt := 0
  for k := 1; ; k++ {
    d := N / k
    cnt += d
    if d <= k { break }
    cnt += k * (d - MaxInt(k + 1, N / (k + 1)))
  }
  Write(cnt)
}

func main() {
  Solve()
  Output()
}

var reader = bufio.NewReaderSize(os.Stdin, 1000000)
var writer = bufio.NewWriter(os.Stdout)
func NextLine() string {
  buffer := make([]byte, 0)
  for true {
    line, isPrefix, err := reader.ReadLine()
    if err != nil { panic(err) }
    buffer = append(buffer, line...)
    if !isPrefix { break }
  }
  return string(buffer)
}
func NextInt(A ...*int) {
  L := strings.Split(NextLine(), " ")
  for i, a := range A {
    *a, _ = strconv.Atoi(L[i])
  }
}
func NextIntSlice(A *[]int) {
  L := strings.Split(NextLine(), " ")
  if len(L) == 1 && L[0] == "" {
    (*A) = make([]int, 0)
    return
  }
  (*A) = make([]int, len(L))
  for i, l := range L {
    (*A)[i], _ = strconv.Atoi(l)
  }
}
func NextFloat(A ...*float64) {
  L := strings.Split(NextLine(), " ")
  for i, a := range A {
    *a, _ = strconv.ParseFloat(L[i], 64)
  }
}
func NextFloatSlice(A *[]float64) {
  L := strings.Split(NextLine(), " ")
  if len(L) == 1 && L[0] == "" {
    (*A) = make([]float64, 0)
    return
  }
  (*A) = make([]float64, len(L))
  for i, l := range L {
    (*A)[i], _ = strconv.ParseFloat(l, 64)
  }
}
func Write(S ...interface{}) {
  fmt.Fprintln(writer, S...)
}
func WriteIntSlice(A []int) {
  B := make([]interface{}, len(A))
  for i, a := range A { B[i] = a }
  fmt.Fprintln(writer, B...)
}
func Output() {
  _ = writer.Flush()
}
func MaxInt(A ...int) int {
  max := A[0]
  for _, a := range A {
    if max < a { max = a }
  }
  return max
}
func MinInt(A ...int) int {
  min := A[0]
  for _, a := range A {
    if a < min { min = a }
  }
  return min
}
func SumInt(A ...int) int {
  sum := 0
  for _, a := range A {
    sum += a
  }
  return sum
}
func AbsInt(x int) int {
  if x < 0 { x = -x }
  return x
}