#include<stdio.h>
int v[100000];
int p[100000];
int nr,n,i,j,k;

int main(){

 FILE *f=fopen("operatii.in","r");

 fscanf(f,"%d",&n);

 for(i=1;i<=n;i++){
    fscanf(f,"%d",&v[i]);
 }

 fclose(f);

nr=0;

  for(i=0;i<=n;i++){

     if(p[i]!=v[i]){

	for(j=i;j<=n+1;j++){

	   if(p[j]==v[j]){

	     for(k=i;k<=j-1;k++){
	     p[k]++;
	     i=0;
	     }
	   nr++;break;
	   }

	}

     }
  }


FILE *g=fopen("operatii.out","w");
fprintf(g,"%d",nr);
fclose(g);

return 0;
}