#include <stdio.h>
#define MAX(x,y) (((x)>(y))?(x):(y))
#define MIN(x,y) (((x)<(y))?(x):(y))
int maxl[10000];
int visine[10000];
int main(void)
{
	int maxdo;
	int sirina;
	int i;
	long voda=0;
	FILE *ulaz,*izlaz;
	ulaz=fopen("kisa.in","r");
	fscanf(ulaz,"%d",&sirina);
	maxdo=0;
	for(i=0;i<sirina;i++){
		fscanf(ulaz,"%d",visine+i);
		maxl[i]=maxdo=MAX(maxdo,visine[i]);
	}
	fclose(ulaz);
	maxdo=0;
	for(i=sirina-1;i>=0;i--){
		maxdo=MAX(maxdo,visine[i]);
		voda+=MIN(maxl[i],maxdo)-visine[i];
	}
	izlaz=fopen("kisa.out","w");
	fprintf(izlaz,"%ld\n",voda);
	return 0;
}