#include <ctype.h>
#include <stdio.h>

int atoi(char s[]);

int main() {
  printf("%d\n", atoi("234"));
}

int atoi(char string[]) {
  int n = 0;
  /*
  * v1
  for (int i = 0; string[i] >= '0' && string[i] <= '9'; ++i)
    n = 10 * n + (string[i] - '0');
  */
  for (int i = 0; isdigit(string[i]); ++i)
    n = 10 * n + (string[i] - '0');

  return n;
}