Description of the meaning of the question: Given an n, find the number of 1-n co-prime numbers.
#include<cstdio>
#include<cmath>
using namespace std;
int res[100000];
int main(){
int n;scanf("%d",&n);
int ans=n;
int tot = 0;
for(int i=2;i*i<n;i++){
if (n% i == 0) res [++ tot] = i;
while(n%i==0)n/=i;
}
if(n>1)res[++tot]=n;
for(int i=1;i<=tot;i++){
ans=ans-ans/res[i];
}
printf("%d\n",ans);
return 0;
}