#include <iostream>
int a[1000];
using namespace std;

int main()
{
int k, n, sum = 0, otvet = 0, cars = 0;
cin >> k >> n;
for (int i = 0; i < n; i++)
{
cin >> a[i];
cars = a[i]-k+a[i+1];
}
cout << cars;
}