N=int(input())
Y=[0]+list(map(int,input().split()))

C=0
for i in range(N-1):
    if Y[i]<=Y[i+1]>Y[i+2]:
        C+=Y[i+1]-Y[i+2]
        if Y[i]>Y[i+2]:
            Y[i+2]=Y[i+1]
        else:
            Y[i+1]=Y[i+2]

print(C)