//Problema operatii
# include <fstream.h>
# include <string.h>
	ifstream fin ("operatii.in");
	ofstream fout ("opeatii.out");
	long n, i, j, x1, x2, k=1, nr;
	long v[1000000], max;
void caut ()
{
	for (i=1; i<=n; i++)
		if (v[i]>max)
			max=v[i];
}
int calcul (int n)
{
	while (k<=max)
	{
		for (i=x2+1; i<=n; i++)
			if (v[i]>=k)
			{
				x1=i;
				nr++;
				break;
			}
		for (j=x1+1; j<=n; j++)
			if (v[j]==k-1)
			{
				x2=j-1;
				break;
			}
		if (j>=n)
			x2=n;
		if (i==n+1)
		{
			k++;
			x1=0;
			x2=0;
		}
	}
	return nr;
}
int main ()
{
	fin >> n;
	for (i=1; i<=n; i++)
		fin >> v[i];
	caut ();
	fout << calcul (n) << '\n';
	return 0;
}