Use of const in c++

Please see this code snippet. Why it is getting compiled and generating output when the value of k is decided at the runtime?

#include <iostream>

using namespace std;

constexpr auto calcfactorialcompiletime(int k) {
    int ans = 1;
    for( int i=1; i <=k; ++i) {
        ans = ans*i;

    }
    return ans;
}

int main() {

    int k;
    cin>>k;
    const auto ans = calcfactorialcompiletime(k);  
    cout<<ans;
    //ans = 9;
    return 0;

}