import std.algorithm, std.container, std.conv, std.math, std.range, std.typecons, std.stdio, std.string;

auto rdsp(){return readln.splitter;}
void pick(R,T)(ref R r,ref T t){t=r.front.to!T;r.popFront;}
void readV(T...)(ref T t){auto r=rdsp;foreach(ref v;t)pick(r,v);}
void readA(T)(size_t n,ref T[]t){t=new T[](n);auto r=rdsp;foreach(ref v;t)pick(r,v);}

void main()
{
  int n; readV(n);
  int[] h; readA(n, h);

  h ~= 0;
  for (auto i = 0; ; ++i) {
    auto l = h.countUntil!"a>0";
    if (l == -1) {
      writeln(i);
      return;
    }
    auto c = h[l..$].countUntil(0);
    --h[l..l+c];
  }    
}
