#include <bits/stdc++.h>
using namespace std;
// type alias
typedef long long LL;
typedef pair<int,int> II;
typedef tuple<int,int,int> III;
typedef vector<int> VI;
typedef vector<string> VS;
typedef unordered_map<int,int> MAPII;
typedef unordered_set<int> SETI;
template<class T> using VV=vector<vector<T>>;
// minmax
template<class T> inline T SMIN(T& a, const T b) { return a=(a>b)?b:a; }
template<class T> inline T SMAX(T& a, const T b) { return a=(a<b)?b:a; }
// repetition
#define FORE(i,a,b) for(int i=(a);i<=(b);++i)
#define REPE(i,n)  for(int i=0;i<=(n);++i)
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  for(int i=0;i<(n);++i)
#define FORR(x,arr) for(auto& x:arr)
#define SZ(a) int((a).size())
// collection
#define ALL(c) (c).begin(),(c).end()
// DP
#define MINUS(dp) memset(dp, -1, sizeof(dp))
#define ZERO(dp) memset(dp, 0, sizeof(dp))
// stdout
#define println(args...) fprintf(stdout, ##args),putchar('\n');
// debug cerr
template<class Iter> void __kumaerrc(Iter begin, Iter end) { for(; begin!=end; ++begin) { cerr<<*begin<<','; } cerr<<endl; }
void __kumaerr(istream_iterator<string> it) { (void)it; cerr<<endl; }
template<typename T, typename... Args> void __kumaerr(istream_iterator<string> it, T a, Args... args) { cerr<<*it<<"="<<a<<", ",__kumaerr(++it, args...); }
template<typename S, typename T> std::ostream& operator<<(std::ostream& _os, const std::pair<S,T>& _p) { return _os<<"{"<<_p.first<<','<<_p.second<<"}"; }
#define __KUMATRACE__ true
#ifdef __KUMATRACE__
#define dump(args...) { string _s = #args; replace(_s.begin(), _s.end(), ',', ' '); stringstream _ss(_s); istream_iterator<string> _it(_ss); __kumaerr(_it, args); }
#define dumpc(ar) { cerr<<#ar<<": "; FORR(x,(ar)) { cerr << x << ','; } cerr << endl; }
#define dumpC(beg,end) { cerr<<"~"<<#end<<": "; __kumaerrc(beg,end); }
#else
#define dump(args...)
#define dumpc(ar)
#define dumpC(beg,end)
#endif

// $ cp-batch SequenceGrowingEasy | diff SequenceGrowingEasy.out -
// $ g++ -std=c++14 -Wall -O2 -D_GLIBCXX_DEBUG -fsanitize=address SequenceGrowingEasy.cpp && ./a.out

/*
 
 4/7/2020
 
 9:39-
 
 */

const int MAX_N=1e6+1;
LL A[MAX_N];
int N;

void nope() {
  cout<<-1<<endl;
  exit(0);
}
void solve() {
  REP(i,N) if(A[i]>i) nope();
  LL res=0;
  int l=0;
  while(l<N) {
    int r=l;
    while(r+1<N&&A[r+1]>A[r]) ++r;
    if(A[r]-A[l]==r-l) {
//      dump(l,r,r-l,A[r]);
      res+=A[r];
    } else {
      nope();
    }
    l=r+1;
  }
  cout<<res<<endl;
}

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout<<setprecision(12)<<fixed;
  
  cin>>N;
  REP(i,N) cin>>A[i];
  solve();
  
  return 0;
}


#include <bits/stdc++.h>
using namespace std;
int main()
{
    int n;
    std::vector<long long> trips;
    int ans=0;
    cin>>n;
    trips.resize(n);
    for(auto &x:trips)
        cin>>x;
    n = trips.size();
    for(int i=0;i<n;i++){
        for(int j=i+1;j<n;j++){
            for(int k=j+1;k<n;k++){
                if(trips[i]!=trips[j] && trips[i]!=trips[k] && trips[j]!=trips[k])
                    if(trips[i]+trips[j]>trips[k] && trips[i]+trips[k]>trips[j] && trips[j]+trips[k]>trips[i])
                        ans++;
            }
        }
    }
    cout << ans;
    return 0;
}
