// #include <ext/pb_ds/assoc_container.hpp> 
                                         // #include <ext/pb_ds/tree_policy.hpp> 
                                            #include <iostream>
                                            #include <iomanip>
                                            #include <sstream>
                                            #include <cstring>
                                            #include <vector>
                                            #include <deque>
                                            #include <queue>
                                            #include <set>
                                            #include <map>
                                            #include <valarray>
                                            #include <iterator>
                                            #include <functional>
                                            #include <limits>
                                            #include <algorithm>
                                            #include <numeric>
                                            #include <cmath>
                                            #include <cassert>
                                            #include <unordered_map>
                                            #include <unordered_set>
                                            #include <stack>
                                            using namespace std;
                                            #define IOS ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
                                             #define trace(x) cerr << #x << ": " << x << " " << endl;
                                         
                                            typedef int64_t ll;
                                         
                                            #define endl '\n'
                                            #define int ll
                                            ll mod=1e9+7;
                                            ll mod1=1e9+6;
                                            
                                            ll power(ll a,ll b)
                                            {
                                                if(b==0) return 1;
                                                else if(b%2==0)
                                                    return power((((a%mod)*(a%mod))%mod),b/2)%mod;
                                                else return ((a%mod)*(power((((a%mod)*(a%mod))%mod),b/2)%mod))%mod;
                                            }
                                           
                                          //  using namespace __gnu_pbds;
                                           // #define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
                                            //find_by_order(k)  returns iterator to kth element starting from 0;
                                            //order_of_key(k) returns count of elements strictly smaller than k;
                                            //erase,insert same as normal set
                                        
                                        int32_t main()
                                        {
                                            IOS
                                         
                                            string s;
                                            cin>>s;
                                            int ans=0;
                                            int te=0;
                                            for(int i=0;i<s.length();++i)
                                                if(s[i]=='A' or s[i]=='T' or s[i]=='C' or s[i]=='G')
                                                    te++;
                                                else
                                                {
                                                    ans=max(ans,te);
                                                    te=0;
                                                }
                                                ans=max(ans,te);
                                                cout<<ans;
                                        
                                        }                   

Measured memory_bytes: 256