#include<bits/stdc++.h>
using namespace std;
long long n,m,a[1005],sum;
int main(){
	cin>>n>>m;
	for(int i = 1;i <= m;i++){
		cin>>a[i];
		if(a[1] > n){
			sum = sum+a[1]-n;
			a[1] = n;
		}else if(a[i]+a[i-1] > n){
			sum = sum+a[i]+a[i-1]-n;
			a[i] = a[i]-(a[i]+a[i-1]-n);
		}
	
	}
	cout<<sum;
}