n=int(input())
a=list(map(int,input().split()))
ans=0
tmp=a[0]
for i in range(n-1):
    x=a[i+1]-a[i]
    if x<0:
        if tmp>a[i+1]:
            z=a[i]-tmp+tmp-a[i+1]
            ans+=z
            a[i]=tmp
            a[i+1]=tmp
        else:
            z=a[i]-a[i+1]
            ans+=z
            a[i]=a[i+1]
    tmp=a[i]
print(ans)