#include <iostream>
using namespace std;

int main()
{
int k, n, a, sum = 0, otvet = 0;
cin >> k >> n;
for (int i = 0; i < n; i++)
{
cin >> a;
sum += a;
}
if (sum < (k * n))
{
cout << 0;
}
else { otvet = sum % (k * n);
cout << otvet;
}
}