Sum of first K numbers which are not divisible by N | C ++ Program to calculate the sum of first K numbers not divisible by N ; Function to find the sum ; Find the last multiple of N ; Find the K - th non - multiple of N ; Calculate the sum of all elements from 1 to val ; Calculate the sum of all multiples of N between 1 to val ; Driver code
#include <bits/stdc++.h> NEW_LINE using namespace std ; int findSum ( int n , int k ) { int val = ( k / ( n - 1 ) ) * n ; int rem = k % ( n - 1 ) ; if ( rem == 0 ) { val = val - 1 ; } else { val = val + rem ; } int sum = ( val * ( val + 1 ) ) / 2 ; int x = k / ( n - 1 ) ; int sum_of_multiples = ( x * ( x + 1 ) * n ) / 2 ; sum -= sum_of_multiples ; return sum ; } int main ( ) { int n = 7 , k = 13 ; cout << findSum ( n , k ) << endl ; }