#include <iostream>
using namespace std;
int main(){
    int x,n;
    cin>>x>>n;
    int p[n+1],a[n+1];
    p[0]=0;
    p[n+1]=0;
    for(int i=1;i<=n;i++){
        a[i]=x;
    }
    for(int i=1;i<=n;i++){
        cin>>p[i];
        a[i]-=p[i];
        a[i+1]+=a[i];
    }
    a[n+1]+=x;
    cout<<a[n+1];
    return 0;
}