#pragma GCC optimize ("Ofast")
#include<bits/stdc++.h>
using namespace std;
void *wmem;
char memarr[96000000];
template<class S, class T> inline S max_L(S a,T b){
  return a>=b?a:b;
}
template<class T> inline void walloc1d(T **arr, int x, void **mem = &wmem){
  static int skip[16] = {0, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1};
  (*mem) = (void*)( ((char*)(*mem)) + skip[((unsigned long long)(*mem)) & 15] );
  (*arr)=(T*)(*mem);
  (*mem)=((*arr)+x);
}
template<class T1> void sortA_L(int N, T1 a[], void *mem = wmem){
  sort(a, a+N);
}
template<class T1, class T2> void sortA_L(int N, T1 a[], T2 b[], void *mem = wmem){
  int i;
  pair<T1, T2> *arr;
  walloc1d(&arr, N, &mem);
  for(i=(0);i<(N);i++){
    arr[i].first = a[i];
    arr[i].second = b[i];
  }
  sort(arr, arr+N);
  for(i=(0);i<(N);i++){
    a[i] = arr[i].first;
    b[i] = arr[i].second;
  }
}
inline int my_getchar_unlocked(){
  static char buf[1048576];
  static int s = 1048576;
  static int e = 1048576;
  if(s == e && e == 1048576){
    e = fread_unlocked(buf, 1, 1048576, stdin);
    s = 0;
  }
  if(s == e){
    return EOF;
  }
  return buf[s++];
}
inline void rd(char &c){
  int i;
  for(;;){
    i = my_getchar_unlocked();
    if(i!=' '&&i!='\n'&&i!='\r'&&i!='\t'&&i!=EOF){
      break;
    }
  }
  c = i;
}
inline int rd(char c[]){
  int i;
  int sz = 0;
  for(;;){
    i = my_getchar_unlocked();
    if(i!=' '&&i!='\n'&&i!='\r'&&i!='\t'&&i!=EOF){
      break;
    }
  }
  c[sz++] = i;
  for(;;){
    i = my_getchar_unlocked();
    if(i==' '||i=='\n'||i=='\r'||i=='\t'||i==EOF){
      break;
    }
    c[sz++] = i;
  }
  c[sz]='\0';
  return sz;
}
struct MY_WRITER{
  char buf[1048576];
  int s;
  int e;
  MY_WRITER(){
    s = 0;
    e = 1048576;
  }
  ~MY_WRITER(){
    if(s){
      fwrite_unlocked(buf, 1, s, stdout);
    }
  }
}
;
MY_WRITER MY_WRITER_VAR;
void my_putchar_unlocked(int a){
  if(MY_WRITER_VAR.s == MY_WRITER_VAR.e){
    fwrite_unlocked(MY_WRITER_VAR.buf, 1, MY_WRITER_VAR.s, stdout);
    MY_WRITER_VAR.s = 0;
  }
  MY_WRITER_VAR.buf[MY_WRITER_VAR.s++] = a;
}
inline void wt_L(char a){
  my_putchar_unlocked(a);
}
inline void wt_L(int x){
  int s=0;
  int m=0;
  char f[10];
  if(x<0){
    m=1;
    x=-x;
  }
  while(x){
    f[s++]=x%10;
    x/=10;
  }
  if(!s){
    f[s++]=0;
  }
  if(m){
    my_putchar_unlocked('-');
  }
  while(s--){
    my_putchar_unlocked(f[s]+'0');
  }
}
template<class S> void arrErase(int k, int &sz, S a[]){
  int i;
  sz--;
  for(i=(k);i<(sz);i++){
    a[i] = a[i+1];
  }
}
template<class S, class T> void arrErase(int k, int &sz, S a[], T b[]){
  int i;
  sz--;
  for(i=(k);i<(sz);i++){
    a[i] = a[i+1];
  }
  for(i=(k);i<(sz);i++){
    b[i] = b[i+1];
  }
}
template<class S, class T, class U> void arrErase(int k, int &sz, S a[], T b[], U c[]){
  int i;
  sz--;
  for(i=(k);i<(sz);i++){
    a[i] = a[i+1];
  }
  for(i=(k);i<(sz);i++){
    b[i] = b[i+1];
  }
  for(i=(k);i<(sz);i++){
    c[i] = c[i+1];
  }
}
template<class S> inline void arrInsert(const int k, int &sz, S a[], const S aval){
  int i;
  sz++;
  for(i=sz-1;i>k;i--){
    a[i] = a[i-1];
  }
  a[k] = aval;
}
template<class S, class T> inline void arrInsert(const int k, int &sz, S a[], const S aval, T b[], const T bval){
  int i;
  sz++;
  for(i=sz-1;i>k;i--){
    a[i] = a[i-1];
  }
  for(i=sz-1;i>k;i--){
    b[i] = b[i-1];
  }
  a[k] = aval;
  b[k] = bval;
}
template<class S, class T, class U> inline void arrInsert(const int k, int &sz, S a[], const S aval, T b[], const T bval, U c[], const U cval){
  int i;
  sz++;
  for(i=sz-1;i>k;i--){
    a[i] = a[i-1];
  }
  for(i=sz-1;i>k;i--){
    b[i] = b[i-1];
  }
  for(i=sz-1;i>k;i--){
    c[i] = c[i-1];
  }
  a[k] = aval;
  b[k] = bval;
  c[k] = cval;
}
template<class S, class T, class U, class V> inline void arrInsert(const int k, int &sz, S a[], const S aval, T b[], const T bval, U c[], const U cval, V d[], const V dval){
  int i;
  sz++;
  for(i=sz-1;i>k;i--){
    a[i] = a[i-1];
  }
  for(i=sz-1;i>k;i--){
    b[i] = b[i-1];
  }
  for(i=sz-1;i>k;i--){
    c[i] = c[i-1];
  }
  for(i=sz-1;i>k;i--){
    d[i] = d[i-1];
  }
  a[k] = aval;
  b[k] = bval;
  c[k] = cval;
  d[k] = dval;
}
int N;
char S[1000000+2];
int ds;
int dp0[10000];
int dp1[10000];
int ns;
int nx0[10000];
int nx1[10000];
int main(){
  wmem = memarr;
  int i;
  int j;
  int k;
  int res;
  N = rd(S);
  int Lj4PdHRW;
  int KL2GvlyY;
  int Q5VJL1cS;
  Lj4PdHRW = 1;
  KL2GvlyY = N;
  while(Lj4PdHRW < KL2GvlyY){
    if((Lj4PdHRW + KL2GvlyY)%2==0){
      Q5VJL1cS = (Lj4PdHRW + KL2GvlyY) / 2;
    }
    else{
      Q5VJL1cS = (Lj4PdHRW + KL2GvlyY - 1) / 2;
    }
    ds = 1;
    dp0[0] = dp1[0] = 0;
    for(k=(0);k<(N);k++){
      ns = 0;
      for(i=(0);i<(ds);i++){
        if(S[k]=='0' || S[k]=='?'){
          nx0[ns] =max_L(0, dp0[i]+1);
          nx1[ns] =max_L(0, dp1[i]-1);
          ns++;
        }
        if(S[k]=='1' || S[k]=='?'){
          nx0[ns] =max_L(0, dp0[i]-1);
          nx1[ns] =max_L(0, dp1[i]+1);
          ns++;
        }
      }
      ds = 0;
      for(i=(0);i<(ns);i++){
        if(nx0[i] <= Q5VJL1cS && nx1[i] <= Q5VJL1cS){
          dp0[ds] = nx0[i];
          dp1[ds] = nx1[i];
          ds++;
        }
      }
      sortA_L(ds, dp0, dp1);
      j = 0;
      for(i=(0);i<(ds);i++){
        if(j && dp1[i] >= dp1[j-1]){
          continue;
        }
        dp0[j] = dp0[i];
        dp1[j] = dp1[i];
        j++;
      }
      ds = j;
      if(ds >= 3 && dp0[1] + 2 < dp0[2]){
        arrInsert(2, ds, dp0, dp0[1]+2, dp1, dp1[1]-2);
      }
      if(ds >= 3 && dp0[ds-3] +2 < dp0[ds-2]){
        arrInsert(ds-2, ds, dp0, dp0[ds-2]-2, dp1, dp1[ds-2]+2);
      }
      while(ds > 6){
        arrErase(3, ds, dp0, dp1);
      }
    }
    if(ds){
      KL2GvlyY = Q5VJL1cS;
    }
    else{
      Lj4PdHRW = Q5VJL1cS + 1;
    }
  }
  res =KL2GvlyY;
  wt_L(res);
  wt_L('\n');
  return 0;
}
// cLay varsion 20200509-1

// --- original code ---
// int N;
// char S[1d6+2];
// 
// int ds, dp0[10000], dp1[10000];
// int ns, nx0[10000], nx1[10000];
// 
// {
//   int i, j, k, res;
//   rd(S@N);
//   // N = 1d6;
//   // rep(i,N) S[i] = rand()%3 + '0';
//   // rep(i,N) if(S[i]=='2') S[i] = '?';
// 
//   res = bsearch_min[int,x,1,N][
//     ds = 1;
//     dp0[0] = dp1[0] = 0;
// 
//     rep(k,N){
//       ns = 0;
//       rep(i,ds){
//         if(S[k]=='0' || S[k]=='?'){
//           nx0[ns] = max(0, dp0[i]+1);
//           nx1[ns] = max(0, dp1[i]-1);
//           ns++;
//         }
//         if(S[k]=='1' || S[k]=='?'){
//           nx0[ns] = max(0, dp0[i]-1);
//           nx1[ns] = max(0, dp1[i]+1);
//           ns++;
//         }
//       }
// 
//       ds = 0;
//       rep(i,ns) if(nx0[i] <= x && nx1[i] <= x){
//         dp0[ds] = nx0[i];
//         dp1[ds] = nx1[i];
//         ds++;
//       }
// 
//       sortA(ds, dp0, dp1);
//       j = 0;
//       rep(i,ds){
//         if(j && dp1[i] >= dp1[j-1]) continue;
//         dp0[j] = dp0[i];
//         dp1[j] = dp1[i];
//         j++;
//       }
//       ds = j;
// 
//       if(ds >= 3 && dp0[1] + 2 < dp0[2]) arrInsert(2, ds, dp0, dp0[1]+2, dp1, dp1[1]-2);
//       if(ds >= 3 && dp0[ds-3] +2 < dp0[ds-2]) arrInsert(ds-2, ds, dp0, dp0[ds-2]-2, dp1, dp1[ds-2]+2);
//       while(ds > 6) arrErase(3, ds, dp0, dp1);
// 
// /*      if(x==6){
//         wtF("{S[k]}");
//         rep(i,ds) wtF(" [{dp0[i]},{dp1[i]}]"); wt("");
//       }*/
//     }
//   ](ds);
// 
//   wt(res);
// }


Measured memory_bytes: 5620