#include <iostream>
#include <algorithm>
using namespace std;

int main() {
  int n;
  cin >> n;
  int b[n];
  for(int i = 0;i < n;i++){
    cin >> b[i];
  }
  int cnt = 0;
  int arr[b[n-1]+2] = {0,};
  for(int i = 0;i < n;i++){
    arr[b[i]] = 67676767;
  }
  arr[0] = 0;
  arr[1] = 1;
for (int i = 0;i < b[n-1]+2;i++) {
    if (arr[i] == 67676767) {
        if (cnt == 1) {
            cout << -1;
            return 0;
        }
        else {
            cnt++;
        }
    }
    else{
        cnt = 0;
    }
}
  for(int i = 2;i < b[n-1]+2;i++){
    if(arr[i] == 67676767){
      continue;
    }
     arr[i] = min(arr[i-1],arr[i-2])+1;
  }
    cout << arr[b[n-1]+1];
  return 0;
}