c_cpp coprimes_pairs

Posted 2021-05-14

#include <bits/stdc++.h>
using namespace std;
#define N 10001
int phi[N];
int S[N];
void coprimes(){
    phi[0] = 1;
    for(int i = 1;i <= N;i++){phi[i] = i;}
    for(int p = 2;p<= N;p++){
    if(phi[p] == p){
        phi[p] = p - 1;
    
    for(int j = 2*p;j <= N;j += p){
          
            phi[j] = (phi[j]*(p - 1))/p;
        }
    }
    }
}

void coprimes_pairs(){
    S[0] = 0;
    for(int i = 1;i <= N;i++){
        S[i] = S[i - 1] + phi[i];
    }
}
int main() {
    int x;
    coprimes();
    coprimes_pairs();
    cin>> x;
    cout << S[x] << endl;
	return 0;
}