#include <stdio.h>
int n,v[1000001],i,j,nr,t;
int main() {
	FILE *f=fopen("operatii.in","r");
	FILE *g=fopen("operatii.out","w");
	fscanf(f,"%d",&n);
	for(i=1;i<=n;i++)
		fscanf(f,"%d",&v[i]);
	for(i=1;i<=n;i++)
		if(v[i]!=0) {
			nr=nr+v[i];
			t=v[i];
			v[i]=0;
			j=i;
			while(v[j+1]>0&&j<n) {
				j++;
				if(j<=n)
					if(v[j]-t>=0)
						v[j]=v[j]-t;
					else
						v[j]=0;
			}
		}
	fprintf(g,"%d",nr);
	return 0;
}