challenge-089/ash/cpp/ch-1.cpp


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// Compile as:
// $ g++ -std=c++17 ch-1.cpp

// Test run:
// $ ./a.out 100
// 13015

#include <iostream>
#include <numeric>
#include <sstream>

using namespace std;

int main(int argc, char** argv) {
    int n = 3;
    if (argc != 1) {
        stringstream input(argv[1]);
        input >> n;
    }

    int s = 0;
    for (int x = 1; x <= n; x++) {
        for (int y = x + 1; y <= n; y++) {
            s += gcd(x, y);
        }
    }

    cout << s << endl;
}