output_description: Print the answer.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      s_{1}s_{2}s_{3}
attempt: import sys
  from collections import defaultdict, Counter
  from itertools import product, groupby, count, permutations, combinations
  from math import pi, sqrt, ceil, floor
  from collections import deque
  from bisect import bisect, bisect_left, bisect_right
  from string import ascii_lowercase
  from functools import lru_cache, reduce
  from operator import xor
  
  INF = float("inf")
  sys.setrecursionlimit(10**7)
  
  # 4近傍（右, 下, 左, 上）
  dy = [0, -1, 0, 1]
  dx = [1, 0, -1, 0]
  
  
  def inside(y: int, x: int, H: int, W: int) -> bool:
      return 0 <= y < H and 0 <= x < W
  
  
  def main():
      s = input()
      print(s.count("1"))
  
  
  if __name__ == "__main__":
      main()
  
problem_description: Statement
  
  Snuke has a grid consisting of three squares numbered 1, 2 and 3. In each
  square, either `0` or `1` is written. The number written in Square i is s_i.
  
  Snuke will place a marble on each square that says `1`. Find the number of
  squares on which Snuke will place a marble.
samples: [{"input": "101", "output": "2\n    \n\n  * A marble will be placed on Square 1 and 3.\n\n* * *"}, {"input": "000", "output": "0\n    \n\n  * No marble will be placed on any square."}]