#include<stdio.h>
#define dim 101

long n,l,ok,nr,i,a[dim][dim],k,j;

int main()
{
	freopen("operatii.in","r",stdin);
	freopen("operatii.out","w",stdout);

	scanf("%ld",&n);

	for(l=1; l<=n; ++l)
	{
		scanf("%ld",&nr);

		if(nr==0 && ok==1)
		{
			a[i][0]=k;

			ok=0;

			j=0;

                        k=0;
		}
		if(nr==0 && ok==0)

			++i;ok=1;

		if(ok==1 && nr)
		{
			a[i][++j]=nr;

			++k;

		}
	}
	a[i][0]=k;

	l=i;

	k=0;

	for(i=1; i<=l; ++i)
	{
		nr=a[i][0];

		while(nr)
		{
			for(j=1; j<=a[i][0]; ++j)
			{
				a[i][j]=a[i][j]-1;

				if(a[i][j]==0)

					--nr;
			}
			++k;
		}
	}
	printf("%ld",k);

	return 0;
}