simple factoring doesn't work

[Arez]

hey what's wrong with this i can't find it

#include <iostream>
using namespace std;

int main()
{
    int a,b,c;
    cout << "Enter a number ";
    cin >> a;
    for (int n=1; n<a; n++)
    {
        b=a*(a-n);
        c=c+b;
    }
    cout << c;
    return 0;
}

its supposed to factor but doesn't!