#include <fstream>

using namespace std;

 ifstream x ("operatii.in");
 ofstream y ("operatii.out");

 int i,ii,j,k,l,n,v[1000005],nr,flag;

 void read()
   {x>>n;
    for(i=1;i<=n;i++)
      x>>v[i];
   }

 void check()
   {flag=1;
    ii=n+1;
    for(l=1;l<=ii--;l++)
      if(v[l]>0 || v[ii]>0)
        {flag=0;
         break;
        }
   }

 void operatii()
   {check();
    i=1;
    while(flag==0)
      {for(i=l;i<=n;i++)
         if(v[i])
           break;
       for(j=i+1;j<=n;j++)
         if(v[j]==0)
           break;
       j=j-1;
       for(k=i;k<=j;k++)
         v[k]--;
       nr++;
       check();
       l=i;
      }
    y<<nr<<'\n';
   }

int main()
{read();
 operatii();
 return 0;
}