#include <stdio.h>
int n,v[1000001],i,j,nr;
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];
			j=i+1;
			while(v[j]!=0) {
				if(v[j]-v[i]>=0)
					v[j]=v[j]-v[i];
				else
					v[j]=0;
				j++;
			}
			v[i]=0;
			i=j-2;
		}
	fprintf(g,"%d",nr);
	return 0;
}