I rewrote my code as follows:

#include <iostream> int main() { // 3 + 6 +....+999 = 3 ( 1 + 2+ ...+333) =3 * ( 333 * 334) unsigned int sum_mult_3_upto_1000 = 3 * 333 * 334 >> 1; unsigned int sum_mult_5_upto_1000 = 5 * 199 * 200 >> 1; unsigned int sum_mult_15_upto_1000 = 15 * 67 * 66 >> 1; unsigned sum = sum_mult_3_upto_1000 + sum_mult_5_upto_1000 - sum_mult_15_upto_1000; std::cout<< sum <<"\n"; }