#include<stdio.h>

int a[100001],i,j,k,n,p,t,rez;

int main()
{ 
    freopen("grupuri.in","r",stdin);
    freopen("grupuri.out","w",stdout);
    
    scanf("%d %d",&k,&n);
    for(i=1;i<=n;i++) scanf("%d",&a[i]);
    //if(k<=n/2);
    for(i=n;i-k+1>=1;) { p+=a[i-k+1]; 
                         a[i-k+1]=0;  
                         for(j=i-k+2;j<=n;j++) if(a[j]-p>0) { p+=a[j]-p;  
                                                            a[j]=0;
                                                            for(t=1;t<=k-(n-j+1);t++) a[i-k+1-t]--;          
                                                          }
                         i=i-k;
                         while(a[i]<=0) i--;
                         rez+=p;
                         p=0;
                       }
  printf("%d\n",rez);
  fclose(stdin);
  fclose(stdout);
  return 0;
}