import sys
input = lambda: sys.stdin.readline().rstrip()

def main():
    # 入力
    S = input()
    # 計算・出力
    RLE = [['0', 0]]
    def RLEappend(s, n):
        if n <= 0: return
        if RLE[-1][0] == s:
            RLE[-1][1] += n
        else:
            RLE.append([s, n])
            
    ans = 0
    for s in S:
        if s == '0' and RLE[-1][0] == '1':
            _, one = RLE.pop()
            d, r = divmod(one, 2)
            ans += d
            RLEappend('1', r)
            RLEappend('0', 1)
            RLEappend('1', 2*d)
        else:
            RLEappend(s, 1)
    print(ans)

if __name__ == "__main__":
    main()