Timus_1787

//Timus 1787
#include <iostream>
using namespace std;
int main()
{
    int k, n, nn[101], z, sum=0;
    cin >> k >> n;
    for(int i=0;i<n;i++)
    {
        cin >> nn[i];
        sum += nn[i];
        sum -= k;
        if(sum<0){sum=0;}
    }
    cout << sum;
    return 0;
}