#include <iostream>

/* run this program using the console pauser or add your own getch, system("pause") or input loop */
using namespace std;
int main(int argc, char *argv[]) {
    int k,n,i;
    cin>>k>>n;
    int y[n+1];
    for (i=1;i<=n;++i)
    {
        cin>>y[i];
    }
    for (i=1;i<=n;++i)
    {
        if (y[i]>k) {y[i+1]+=y[i]-k;}
        else {y[i]=0;}
    }
    cout<<y[n+1]<<endl;
    return 0;
}