import sys
sys.setrecursionlimit(10**6)

def calc( n , m , N , S ) :
    if n == N :
        return 0
    elif n == N-1 :
        if S[n] == "1" :
            return 0
        else :
            return m
    else :
        if S[n] == "1" :
            if S[n+1] == "1" :
                return calc( n+2 , m+1 , N , S )
            else :
                return m + calc( n+2 , m , N , S )
        else :
            return m + calc( n+1 , m , N , S )

def main() :
    S = list( input() )
    N = len(S)
    print( calc( 0 , 0 , N , S ) )

main()