#include <stdio.h>
#include <string.h>

#define min(x,y) ((x)<(y)?(x):(y))

double htx[6000];
#define ht (htx+3000)

int leftx, rightx, L,R,i,j,k,ii,jj;
double rheight,lheight,rneed,lneed,need, H,vol;

main(){
   while (2 == scanf("%d%d",&leftx,&rightx) && leftx) {
      memset(htx,0,sizeof(htx));
      vol = 0;
      ht[leftx-2] = ht[rightx+2] = 0x7fffffff;
      for (i=leftx;i<=rightx;i+=2) scanf("%lf",&ht[i]);
      L = -1;  R = 1;
      while (ht[leftx] > ht[leftx+1] && ht[rightx] > ht[rightx-1]) {
         H = min(ht[L],ht[R]);
         if (ht[L+1] < H || ht[R-1] < H) {
            for (i=L+1;i<=R-1;i+=2) {
               vol += 2 * (H - ht[i]);
               ht[i] = H;
            }
            continue;
         }
         if (ht[R-1] == ht[R]) {
            for (i=R-1;ht[i] >= ht[i+1];i+=2);
            for (j=i;ht[i] == ht[j];j-=2);
            rheight = min(ht[i+1],ht[0]);
            rheight = min(rheight,ht[j+1]);
            rneed = (rheight - ht[i]) * (i-j);
         } else rheight = rneed = 0;
         if (ht[L+1] == ht[L]) {
            for (ii=L-1;ht[ii] >= ht[ii-1];ii-=2);
            for (jj=ii;ht[ii] == ht[jj];jj+=2);
            lheight = min(ht[ii-1],ht[0]);
            lheight = min(lheight,ht[jj-1]);
            lneed = (lheight - ht[ii]) * (jj-ii);
         } else lneed = lheight = 0;
         if (rneed) for (k=j+2;k<=i;k+=2) {
            if (lneed && lneed < rneed) 
               ht[k] += (rheight-ht[i]) * lneed/rneed;
            else
               ht[k] = rheight;
         }
         if (lneed) for (k=jj-2;k>=ii;k-=2) {
            if (rneed && rneed < lneed) 
                ht[k] +=  (lheight-ht[ii]) * rneed/lneed;
            else 
                ht[k] = lheight;
         }
         if (lneed && rneed) vol += 2 * min(lneed,rneed);
         else vol +=  (lneed+rneed);
         while (ht[R+1] == ht[R-1]) R+=2;
         while (ht[L-1] == ht[L+1]) L-=2;
      }
      printf("%0.0lf\n",vol);
   }
}