n=int(input())
ppl=(input().split())
answer=0
for x in range(0,n-1):
    ppl[x]=int(ppl[x])
    ppl[x+1]=int(ppl[x+1])
    if ppl[x]>ppl[x+1]:
        answer+=ppl[x]-ppl[x+1]
        ppl[x+1]=ppl[x]
answer