#include <bits/stdc++.h>
using namespace std;
long long dx[] = {-1, 0, 1, 0};
long long dy[] = {0, 1, 0, -1};
long long spf[1100];
void sieve() {
  long long i, j;
  for (i = 0; i < 1100; i++) spf[i] = i;
  for (i = 4; i < 1100; i += 2) spf[i] = 2;
  for (i = 3; i * i < 1100; i++) {
    if (spf[i] == i) {
      for (j = i * i; j < 1100; j += i) {
        if (spf[j] == j) spf[j] = i;
      }
    }
  }
}
bool isvowel(char c) {
  switch (c) {
    case 'a':
    case 'e':
    case 'i':
    case 'o':
    case 'u':
    case 'y':
      return true;
    default:
      return false;
  }
}
bool cmp(pair<long long, long long> x, pair<long long, long long> y) {
  return x.first > y.first;
}
long long isPrime(long long n) {
  for (long long i = 2; i <= sqrt(n); i++) {
    if (n % i == 0) {
      return i;
    }
  }
  return n;
}
long long sum(long long n) {
  long long ans = 0;
  while (n > 0) {
    long long d = n % 10;
    ans += d;
    n /= 10;
  }
  return ans;
}
vector<long long> adj[5001];
vector<long long> adj1[5001];
long long vis[5001];
long long dist[5001];
long long dist1[5001];
long long vis1[5001];
long long cnt;
void bfs(long long node) {
  vis[node] = 1;
  queue<long long> q;
  q.push(node);
  dist[node] = 0;
  while (!q.empty()) {
    long long curr = q.front();
    q.pop();
    for (auto child : adj[curr]) {
      if (!vis[child]) {
        vis[child] = 1;
        dist[child] = dist[curr] + 1;
        q.push(child);
      }
    }
  }
}
void dfs(long long node) {
  vis1[node] = 1;
  cnt++;
  for (auto child : adj[node]) {
    if (!vis1[child] && !vis[child]) {
      dfs(child);
    }
  }
}
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long t;
  t = 1;
  while (t--) {
    string s;
    cin >> s;
    long long n = s.length();
    long long cnt = 0, cnt1 = 0;
    long long sum = 0;
    long long i = 0;
    while (i < n) {
      long long flag = 0;
      long long x = s[i] - '0';
      if (x % 3 == 0) {
        flag = 1;
        sum++;
        i++;
      } else if (x % 3 == 1) {
        long long y = s[i + 1] - '0';
        if (y % 3 == 2) {
          flag = 1;
          sum++;
          i += 2;
        } else if (y % 3 == 1) {
          long long z = s[i + 2] - '0';
          if (z % 3 == 1) {
            flag = 1;
            sum++;
            i += 3;
          }
        }
      } else if (x % 3 == 2) {
        long long y = s[i + 1] - '0';
        if (y % 3 == 1) {
          sum++;
          flag = 1;
          i += 2;
        } else if (y % 3 == 2) {
          long long z = s[i + 2] - '0';
          if (z % 3 == 2) {
            sum++;
            flag = 1;
            i += 3;
          }
        }
      }
      if (flag == 0) {
        i++;
      }
    }
    cout << sum << "\n";
  }
  return 0;
}
