/*{{{*/
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<string>
#include<iostream>
#include<sstream>
#include<set>
#include<map>
#include<queue>
#include<bitset>
#include<vector>
#include<limits.h>
#include<assert.h>
#define SZ(X) ((int)(X).size())
#define ALL(X) (X).begin(), (X).end()
#define REP(I, N) for (int I = 0; I < (N); ++I)
#define REPP(I, A, B) for (int I = (A); I < (B); ++I)
#define FOR(I, A, B) for (int I = (A); I <= (B); ++I)
#define FORS(I, S) for (int I = 0; S[I]; ++I)
#define RS(X) scanf("%s", (X))
#define SORT_UNIQUE(c) (sort(c.begin(),c.end()), c.resize(distance(c.begin(),unique(c.begin(),c.end()))))
#define GET_POS(c,x) (lower_bound(c.begin(),c.end(),x)-c.begin())
#define CASET int ___T; scanf("%d", &___T); for(int cs=1;cs<=___T;cs++)
#define MP make_pair
#define PB push_back
#define MS0(X) memset((X), 0, sizeof((X)))
#define MS1(X) memset((X), -1, sizeof((X)))
#define LEN(X) strlen(X)
#define F first
#define S second
using namespace std;
typedef long long LL;
typedef unsigned long long ULL;
typedef long double LD;
typedef pair<int,int> PII;
typedef vector<int> VI;
typedef vector<LL> VL;
typedef vector<PII> VPII;
typedef pair<LL,LL> PLL;
typedef vector<PLL> VPLL;
template<class T> void _R(T &x) { cin >> x; }
void _R(int &x) { scanf("%d", &x); }
void _R(int64_t &x) { scanf("%lld", &x); }
void _R(double &x) { scanf("%lf", &x); }
void _R(char &x) { scanf(" %c", &x); }
void _R(char *x) { scanf("%s", x); }
void R() {}
template<class T, class... U> void R(T &head, U &... tail) { _R(head); R(tail...); }
template<class T> void _W(const T &x) { cout << x; }
void _W(const int &x) { printf("%d", x); }
void _W(const int64_t &x) { printf("%lld", x); }
void _W(const double &x) { printf("%.16f", x); }
void _W(const char &x) { putchar(x); }
void _W(const char *x) { printf("%s", x); }
template<class T,class U> void _W(const pair<T,U> &x) {_W(x.F); putchar(' '); _W(x.S);}
template<class T> void _W(const vector<T> &x) { for (auto i = x.begin(); i != x.end(); _W(*i++)) if (i != x.cbegin()) putchar(' '); }
void W() {}
template<class T, class... U> void W(const T &head, const U &... tail) { _W(head); putchar(sizeof...(tail) ? ' ' : '\n'); W(tail...); }
#ifdef HOME
 #define DEBUG(...) {printf("# ");printf(__VA_ARGS__);puts("");}
#else
 #define DEBUG(...)
#endif
int MOD = 1e9+7;
void ADD(LL& x,LL v){x=(x+v)%MOD;if(x<0)x+=MOD;}
/*}}}*/
const int SIZE = 1e6+10;
int N,a[SIZE];
void NO(){
    W(-1);
    exit(0);
}
int mi[SIZE],ma[SIZE];
template <class T>
void maa(T& x,T y){
    if(x<y)x=y;
}
template <class T>
void mii(T& x,T y){
    if(x>y)x=y;
}
struct BIT{
    int _n;
    LL _d[SIZE];
    void init(int __n){
        _n=__n;
        memset(_d,0,sizeof(LL)*(_n+1));
    }
    void ins(int x,LL v){
        for(;x<=_n;x+=x&-x)_d[x]+=v;
    }
    LL qq(int x){
        LL res=0;
        for(;x;x-=x&-x)res+=_d[x];
        return res;
    }
    LL qq_range(int x,int y){
        LL ret=qq(y);
        if(x)ret-=qq(x-1);
        return ret;
    }
}bit;
//TODO init
int main(){
    R(N);
    FOR(i,1,N){
        R(a[i]);
        mi[i]=N+1;
        ma[i]=0;
    }
    bit.init(N);
    if(a[1])NO();
    FOR(i,1,N){
        if(i-a[i]<=0)NO();
        mii(mi[i-a[i]],i);
        maa(ma[i-a[i]],i);
    }
    LL an=0;
    VI d(N);
    REP(i,N)d[i]=i+1;
    set<int>H(ALL(d));
    for(int i=N;i>0;i--){
        if(mi[i]>N)continue;
        an+=ma[i]-i;
        if(bit.qq_range(mi[i],ma[i]))NO();
        auto it=H.lower_bound(mi[i]);
        auto it2=it;
        while(it2!=H.end()&&*it2<=ma[i]){
            bit.ins(*it2,1);
            it2++;
        }
        H.erase(it,it2);
    }
    W(an);
    return 0;
}


Measured memory_bytes: 24320