#pragma GCC optimize ("O3")
#include "bits/stdc++.h"

using namespace std;
using ll = long long int;

#define debug(v) {printf("L%d %s > ",__LINE__,#v);cout<<(v)<<endl;}
#define debugv(v) {printf("L%d %s > ",__LINE__,#v);for(auto e:(v)){cout<<e<<" ";}cout<<endl;}
#define debuga(m,w) {printf("L%d %s > ",__LINE__,#m);for(int x=0;x<(w);x++){cout<<(m)[x]<<" ";}cout<<endl;}
#define debugaa(m,h,w) {printf("L%d %s >\n",__LINE__,#m);for(int y=0;y<(h);y++){for(int x=0;x<(w);x++){cout<<(m)[y][x]<<" ";}cout<<endl;}}
#define ALL(v) (v).begin(),(v).end()
#define repeat(cnt,l) for(remove_const<decltype(l)>::type cnt=0;(cnt)<(l);++(cnt))
#define rrepeat(cnt,l) for(auto cnt=(l)-1;0<=(cnt);--(cnt))
#define iterate(cnt,b,e) for(auto cnt=(b);(cnt)!=(e);++(cnt))
#define diterate(cnt,b,e) for(auto cnt=(b);(cnt)!=(e);--(cnt))
const ll MD = 1000000007ll; const long double PI = 3.1415926535897932384626433832795L;
inline void assert_call(bool assertion, function<void()> f) { if (!assertion) { cerr << "assertion fault:" << endl; f(); abort(); } }
template<typename T1, typename T2> inline ostream& operator <<(ostream &o, const pair<T1, T2> p) { o << '(' << p.first << ':' << p.second << ')'; return o; }
template<typename Vec> inline ostream& _ostream_vecprint(ostream &o, const Vec& p) { o << '['; for (auto& e : p) o << e << ','; o << ']'; return o; }
template<typename T> inline ostream& operator<<(ostream& o, const vector<T>& v) { return _ostream_vecprint(o, v); }
template<typename T, size_t S> inline ostream& operator<<(ostream& o, const array<T, S>& v) { return _ostream_vecprint(o, v); }
template<typename T> inline T& maxset(T& to, const T& val) { return to = max(to, val); }
template<typename T> inline T& minset(T& to, const T& val) { return to = min(to, val); }
void bye(string s, int code = 0) { cout << s << endl; exit(code); }
mt19937_64 randdev(8901016);
template<typename T> inline T rand(T l, T h) { return uniform_int_distribution<T>(l, h)(randdev); }
template<> inline double rand<double>(double l, double h) { return uniform_real_distribution<double>(l, h)(randdev); }
template<> inline float rand<float>(float l, float h) { return uniform_real_distribution<float>(l, h)(randdev); }

#if defined(_WIN32) || defined(_WIN64)
#define getchar_unlocked _getchar_nolock
#define putchar_unlocked _putchar_nolock
#elif defined(__GNUC__)
#else
#define getchar_unlocked getchar
#define putchar_unlocked putchar
#endif
namespace {
#define isvisiblechar(c) (0x21<=(c)&&(c)<=0x7E)
    class MaiScanner {
    public:
        template<typename T> void input_integer(T& var) {
            var = 0; T sign = 1;
            int cc = getchar_unlocked();
            for (; cc<'0' || '9'<cc; cc = getchar_unlocked())
                if (cc == '-') sign = -1;
            for (; '0' <= cc && cc <= '9'; cc = getchar_unlocked())
                var = (var << 3) + (var << 1) + cc - '0';
            var = var * sign;
        }
        inline int c() { return getchar_unlocked(); }
        inline MaiScanner& operator>>(int& var) { input_integer<int>(var); return *this; }
        inline MaiScanner& operator>>(long long& var) { input_integer<long long>(var); return *this; }
        inline MaiScanner& operator>>(string& var) {
            int cc = getchar_unlocked();
            for (; !isvisiblechar(cc); cc = getchar_unlocked());
            for (; isvisiblechar(cc); cc = getchar_unlocked())
                var.push_back(cc);
            return *this;
        }
        template<typename IT> void in(IT begin, IT end) { for (auto it = begin; it != end; ++it) *this >> *it; }
    };
    class MaiPrinter {
    public:
        template<typename T>
        void output_integer(T var) {
            if (var == 0) { putchar_unlocked('0'); return; }
            if (var < 0)
                putchar_unlocked('-'),
                var = -var;
            char stack[32]; int stack_p = 0;
            while (var)
                stack[stack_p++] = '0' + (var % 10),
                var /= 10;
            while (stack_p)
                putchar_unlocked(stack[--stack_p]);
        }
        inline MaiPrinter& operator<<(char c) { putchar_unlocked(c); return *this; }
        inline MaiPrinter& operator<<(int var) { output_integer<int>(var); return *this; }
        inline MaiPrinter& operator<<(long long var) { output_integer<long long>(var); return *this; }
        inline MaiPrinter& operator<<(char* str_p) { while (*str_p) putchar_unlocked(*(str_p++)); return *this; }
        inline MaiPrinter& operator<<(const string& str) {
            const char* p = str.c_str();
            const char* l = p + str.size();
            while (p < l) putchar_unlocked(*p++);
            return *this;
        }
        template<typename IT> void join(IT begin, IT end, char sep = ' ') { for (bool b = 0; begin != end; ++begin, b = 1) b ? *this << sep << *begin : *this << *begin; }
    };
}
MaiScanner scanner;
MaiPrinter printer;

template<typename T>
//typedef ll T;
class SegmentTree {
    int size_;
    vector<T> valdata_;
    vector<T> addtree_;
    vector<T> maxtree_;

    int _geti(const vector<T>& v, int idx) const {
        if (size_ - 1 <= idx) return (idx - size_ + 1);
        // if (idx < 0) return idx;
        return v[idx];
    }
public:

    SegmentTree(int n) {
        size_ = 8;
        while (size_ < n) size_ <<= 1;
        maxtree_.resize(size_);
        valdata_.resize(size_);
        addtree_.resize(size_);
        flat();
    }

    // tree初期化
    void flat() {
        for (int i = size_ - 2; 0 <= i; --i) {
            int c = 2 * i + 1;
            if (size_ - 1 <= c) {
                maxtree_[i] = c - size_ + 1;
            }
            else {
                maxtree_[i] = maxtree_[c];
            }
        }
    }

    // addtreeを0にして，valdataに加算する．
    // void apply(T sum = 0, int ptr = 0) {
    //     if (size_ - 1 <= ptr) {
    //         valdata_[ptr - size_ + 1] += sum;
    //     }
    //     else {
    //         sum += addtree_[ptr];
    //         apply(sum, ptr * 2 + 1);
    //         apply(sum, ptr * 2 + 2);
    //         addtree_[ptr] = 0;
    //     }
    // }

    void fill(T val) {
        std::fill(valdata_.begin(), valdata_.end(), val);
        std::fill(addtree_.begin(), addtree_.end(), 0);
        flat();
    }

    // T operator[](int index) const {
    //     
    // }

    T get_val(int index) const {
        T sum = valdata_[index];
        index += size_ - 1;
        while (0 < index) {
            index = (index - 1) / 2; // 親へ移動
            sum += addtree_[index];
        }
        return sum;
    }

    void set_val(int index, const T e) {
        valdata_[index] = e;

        for (int i = index + size_ - 1; 0 < i;) {
            i = (i - 1) / 2; // 親へ移動
            valdata_[index] -= addtree_[i];
        }
        index += size_ - 1;
        while (0 < index) {
            index = (index - 1) / 2; // 親へ移動
            int c1 = index * 2 + 1;
            int c2 = index * 2 + 2;
            int i1, i2;
            i1 = _geti(maxtree_, c1);
            i2 = _geti(maxtree_, c2);
            maxtree_[index] = get_val(i1) > get_val(i2) ? i1 : i2;
        }
    }

    void _add_valrange(int begin, int end, const T e, int ptr, int rangebegin, int rangeend) {

        if (rangeend <= begin || end <= rangebegin) return;
        if (begin <= rangebegin && rangeend <= end) {
            if (size_ - 1 <= ptr)
                valdata_[ptr - size_ + 1] += e;
            else
                addtree_[ptr] += e;
            return;
        }

        int rangemid = (rangebegin + rangeend) / 2;
        _add_valrange(begin, end, e, ptr * 2 + 1, rangebegin, rangemid);
        _add_valrange(begin, end, e, ptr * 2 + 2, rangemid, rangeend);

        int i1, i2;
        i1 = _geti(maxtree_, ptr * 2 + 1);
        i2 = _geti(maxtree_, ptr * 2 + 2);
        maxtree_[ptr] = get_val(i1) > get_val(i2) ? i1 : i2;
    }

    int _get_maxrangeIdx(int begin, int end, int ptr, int rangebegin, int rangeend) const {

        if (rangeend <= begin || end <= rangebegin) return begin; // note:範囲外なら適当な範囲内のindexを返す
        if (begin <= rangebegin && rangeend <= end) return _geti(maxtree_, ptr);

        int rangemid = (rangebegin + rangeend) / 2;
        int l = _get_maxrangeIdx(begin, end, ptr * 2 + 1, rangebegin, rangemid);
        int r = _get_maxrangeIdx(begin, end, ptr * 2 + 2, rangemid, rangeend);

        if (get_val(l) >= get_val(r))
            return l;
        else
            return r;
    }

    inline void add_valrange(int begin, int end, const T e) {
        _add_valrange(begin, end, e, 0, 0, size_);
    }

    inline int get_maxrangeIdx(int begin, int end) const {
        return _get_maxrangeIdx(begin, end, 0, 0, size_);
    }

    inline T get_maxrange(int begin, int end) {
        return get_val(_get_maxrangeIdx(begin, end, 0, 0, size_));
    }
};


template<typename T = ll>
class Bitree {
public:
    int size_;
    vector<T> data_;
    Bitree(int size_) :size_(size_), data_(size_ + 1) {}

    // 1..rの範囲の値の和を求める．
    T sum(int r) {
        T s = 0;
        while (r) {
            s += data_[r];
            r -= r & -r; //  r&-r と書くことで最下のビットを得る
        }
        return s;
    }
    // l..rの範囲の値の和を求める
    inline T sum(int l, int r) { return sum(r) - sum(l - 1); }
    // idxの要素の値をval増やす
    void add(int idx, T val) {
        while (idx <= size_) {
            data_[idx] += val;
            idx += idx & -idx;
        }
    }
};


/*

初めは xx = [0,-1,-2,...]
次の操作が出来る
xx[i+1] = xx[i]

*/

int N;

ll aa[200010];

int ans[200010];

int main() {
    scanner >> N;
    scanner.in(aa, aa + N);

    if (N == 1) {
        cout << (aa[0] == 0 ? 0 : -1) << endl;
        return 0;
    }

    repeat(i, N+2) {
        aa[i] -= i;
        if (aa[i] > 0) bye("-1");
    }

    ll ans = 0;
    repeat(i, N+1) {
        if (aa[i] < aa[i + 1]) bye("-1");
        if (aa[i] > aa[i + 1]) ans += aa[i] + i;
    }

    cout << ans << endl;


    return 0;
}

Measured memory_bytes: 1792