#pragma GCC optimize ("Ofast")
#include<bits/stdc++.h>
using namespace std;
void *wmem;
char memarr[96000000];
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);
}
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 moddw_L(S a, const T b){
  a %= b;
  if(a < 0){
    a += b;
  }
  return a;
}
template<class S, class T> inline S chmin(S &a, T b){
  if(a>b){
    a=b;
  }
  return a;
}
template<class S, class T> inline S chmax(S &a, T b){
  if(a<b){
    a=b;
  }
  return a;
}
template<class T> void arrRot(int k, int N, T A[], T B[] = NULL, void *mem = wmem){
  int i;
  int fg = 0;
  (k = moddw_L(k,N));
  if(B==NULL){
    walloc1d(&B, N, &mem);
    fg = 1;
  }
  for(i=(k);i<(N);i++){
    B[i-k] = A[i];
  }
  for(i=(0);i<(k);i++){
    B[N-k+i] = A[i];
  }
  if(fg){
    for(i=(0);i<(N);i++){
      A[i] = B[i];
    }
  }
}
int N;
int A[100000];
int mn[100000];
int mx[100000];
int cnt[100000];
int main(){
  int loop;
  wmem = memarr;
  rd(N);
  {
    int Lj4PdHRW;
    for(Lj4PdHRW=(0);Lj4PdHRW<(N);Lj4PdHRW++){
      rd(A[Lj4PdHRW]);A[Lj4PdHRW] += (-1);
    }
  }
  for(loop=(0);loop<(2);loop++){
    int i;
    for(i=(0);i<(N);i++){
      mn[i] = 1073709056;
      mx[i] = -1073709056;
      cnt[i] = 0;
    }
    for(i=(0);i<(N);i++){
      chmin(mn[A[i]], i);
      chmax(mx[A[i]], i);
      cnt[A[i]]++;
    }
    for(i=(0);i<(N);i++){
      if(cnt[i] && mx[i] - mn[i] + 1 != cnt[i]){
        break;
      }
    }
    if(i==N){
      wt_L(loop);
      wt_L('\n');
      return 0;
    }
    for(i=(1);i<(N);i++){
      if(A[i] != A[i-1]){
        break;
      }
    }
    arrRot(i, N, A);
  }
  wt_L(-1);
  wt_L('\n');
  return 0;
}
// cLay varsion 20200509-1

// --- original code ---
// int N, A[1d5];
// int mn[1d5], mx[1d5], cnt[1d5];
// {
//   rd(N,(A--)(N));
//   rep(loop,2){
//     rep(i,N) mn[i] = int_inf, mx[i] = -int_inf, cnt[i] = 0;
//     rep(i,N){
//       mn[A[i]] <?= i;
//       mx[A[i]] >?= i;
//       cnt[A[i]]++;
//     }
//     rep(i,N) if(cnt[i] && mx[i] - mn[i] + 1 != cnt[i]) break;
//     if(i==N) wt(loop), return 0;
// 
//     rep(i,1,N) if(A[i] != A[i-1]) break;
//     arrRot(i, N, A);
//   }
//   wt(-1);
// }