N = int(input(''))
A = input('')
A_N = A.split()
num = 0 
for i in range(1,N):
    if int(A_N[i]) >= int(A_N[i-1]):
        num += 0
    elif (A_N[i]) < (A_N[i-1]):
        num += int(A_N[i-1]) - int(A_N[i])
print(num)