B.cpp

#include<bits/stdc++.h>
#define vi vector<int>
#define mp map<int,int>
#define ump unordered_map <int,int>
#define ll long long int
#define pb push_back
#define mod 1000000007
#define int long long
#define  loop(i,x,n) for(int i=x;i<n;i++)
#define rloop(i,n) for(int i=n-1;i>=0;i--)
using namespace std;
/*************************************************************************************************/
                                                                                                   
signed main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    int n,d;
    cin>>d>>n;
    int a[n];
    loop(i,0,n)
    {
        cin>>a[i];
        a[i]--;
    }
    int day=0,ans=0;
    loop(i,0,n)
    {
        if(day==0)
        {
            day=(a[i]+1)%d;
        }
        else
        {
            ans+=d-day;
            day=(a[i]+1)%d;
        }
        
       // cout<<ans<<' '<<day<<endl;
    }
    cout<<ans;
    return 0;
}