Why does adding unsigned long long int input returns wrong result with just one variable ?

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C++

algorithm

I'm doing a very simple program to add large sum of numbers, I'm using unsigned long long int. I have a working solution already, I just need help to understand why my first trial with just one variable (sum) won't work.

What I have tried:

#include <iostream>
using namespace std;

int main(){
    int input;
    cin >> input; // How many input for sum ?

    // just one variable that I use as input
    // then I just sum this variable via loop
    unsigned long long int sum = 0;

    for (int i =0; i < input; i++){ //loop for sum
        cin >> sum;
        sum+=sum;
    }
    
    cout << sum << endl;
    return 0;
}

The working solution is this one, I've added a new variable for looping input :

#include <iostream>
using namespace std;

int main(){
    int input;
    cin >> input; // How many input for sum ?
    
    //two variables, one for input and one for sum of the input
    unsigned long long int sum = 0, in;

    for (int i =0; i < input; i++){ //loop for sum
        cin >> in;
        sum+=in;
    }
    
    cout << sum << endl;
    return 0;
}

Sample input :

5 (How many input)
1000000001 1000000002 1000000003 1000000004 1000000005 (Actual input)

Answer from code 1          = 2000000010
Correct answer (code 2)     = 5000000015

What explanation did I miss here ?