#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#pragma GCC optimize("inline")
#include<bits/stdc++.h>
using namespace std;
template<class T> struct cLtraits_identity{
  using type = T;
}
;
template<class T> using cLtraits_try_make_signed =
  typename conditional<
    is_integral<T>::value,
    make_signed<T>,
    cLtraits_identity<T>
    >::type;
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(int &x){
  int k;
  int m=0;
  x=0;
  for(;;){
    k = my_getchar_unlocked();
    if(k=='-'){
      m=1;
      break;
    }
    if('0'<=k&&k<='9'){
      x=k-'0';
      break;
    }
  }
  for(;;){
    k = my_getchar_unlocked();
    if(k<'0'||k>'9'){
      break;
    }
    x=x*10+k-'0';
  }
  if(m){
    x=-x;
  }
}
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, class T> inline S chmin(S &a, T b){
  if(a>b){
    a=b;
  }
  return a;
}
int N;
int A[5001];
int sz;
int x[5001];
int y[5001];
int dp[5001];
int main(){
  int k;
  int i;
  int s;
  rd(N);
  {
    int Lj4PdHRW;
    for(Lj4PdHRW=(0);Lj4PdHRW<(N);Lj4PdHRW++){
      rd(A[Lj4PdHRW]);
    }
  }
  A[N++] = 2;
  for(i=(0);i<(N);i++){
    if(A[i]==0){
      x[sz]++;
      y[sz]++;
    }
    if(A[i]==1){
      y[sz]++;
    }
    if(A[i]==2){
      sz++;
    }
  }
  int RZTsC2BF;
  cLtraits_try_make_signed<remove_reference<decltype((*((int*)NULL)))>::type>::type FmcKpFmN;
  if(sz==0){
    FmcKpFmN = 0;
  }
  else{
    FmcKpFmN = x[0];
    for(RZTsC2BF=(1);RZTsC2BF<(sz);RZTsC2BF++){
      FmcKpFmN += x[RZTsC2BF];
    }
  }
  s =FmcKpFmN;
  for(i=(1);i<(s+1);i++){
    dp[i] = 1073709056;
  }
  for(k=(0);k<(sz);k++){
    for(i=(s+1)-1;i>=(y[k]);i--){
      chmin(dp[i], dp[i-y[k]] + (y[k] - x[k]));
    }
  }
  if(dp[s]>=1073709056){
    wt_L(-1);
    wt_L('\n');
  }
  else{
    wt_L(dp[s]);
    wt_L('\n');
  }
  return 0;
}
// cLay version 20210708-1

// --- original code ---
// int N, A[5001];
// int sz, x[], y[], dp[];
// {
//   int i, s;
//   rd(N,A(N));
//   A[N++] = 2;
//   rep(i,N){
//     if(A[i]==0) x[sz]++, y[sz]++;
//     if(A[i]==1) y[sz]++;
//     if(A[i]==2) sz++;
//   }
//   s = sum(x(sz));
//   rep(i,1,s+1) dp[i] = int_inf;
//   rep(k,sz) rrep(i,y[k],s+1) dp[i] <?= dp[i-y[k]] + (y[k] - x[k]);
//   wt(if[dp[s]>=int_inf, -1, dp[s]]);
// }