Function to calculate the sum of numbers divisible by X or Y
int sum ( int N , int X , int Y ) { int S1 , S2 , S3 ; S1 = ( ( N / X ) ) * ( 2 * X + ( N / X - 1 ) * X ) / 2 ; S2 = ( ( N / Y ) ) * ( 2 * Y + ( N / Y - 1 ) * Y ) / 2 ; S3 = ( ( N / ( X * Y ) ) ) * ( 2 * ( X * Y ) + ( N / ( X * Y ) - 1 ) * ( X * Y ) ) / 2 ; return S1 + S2 - S3 ; }