Sum of ones

cc1plus: error: '::main' must return 'int'

#include <iostream>
#include <vector>
#include <string>
#include <math.h>
using namespace std;

long long largest_power(long long n){
    long long x = 0;

    while ((1 << x) <= n)
        ++x;
    return x-1;
}

long long count(long long n){
    
    if (n <= 1)
        return n;

    long long x = largest_power(n);

    return (x * pow(2, (x-1)) + (n- pow(2, x) +1) + count(n - pow(2, x)));
}

long long main() {
    long long n;
    cin >> n;
    
    cout << count(n) << endl;

}