#include <fstream.h>
#define NMAX 1000002
long n,v[NMAX];
int main()
{ long pr,ult,i;
  long long nr=0;
  ifstream f("operatii.in");
  ofstream g("operatii.out");
  f>>n;
  for (i=1;i<=n;i++) f>>v[i];
  f.close();
  pr=ult=1;
  while (pr<=n)
	{ while (v[pr]==0&&pr<=n) pr++;
	  if (pr>n) break;
	  pr--;
	  ult=pr+1;
	  while (v[ult]) ult++;
	  nr++;
	  for (i=pr+1;i<ult;i++) v[i]--;
	}
  g<<nr;
  g.close();
  return 0;
}