#pragma GCC optimize("Ofast,unroll-loops")
//#pragma GCC optimize("conserve-stack")
#include<bits/stdc++.h>
#define int long long
#define ll long long
#define down cout<<'\n';
#define debug cout<<" cucuucucuuu",down
#define NHP ios_base::sync_with_stdio(0);cout.tie(0);cin.tie(0);
#define modwwe int t;cin>>t; while(t--)
#define bit(i,j) (i>>j&1)
#define sobit(a) __builtin_popcountll(a)
#define task "test"
#define fin(x) freopen(x".inp","r",stdin)
#define fou(x) freopen(x".ans","w",stdout)
#define pb push_back
#define mask(i) (1<<i)
#define checktime cerr << (double)clock() / CLOCKS_PER_SEC * 1000 << " ms";
using namespace std;
#define getchar_unlocked getchar
inline int scan()
{
char c = getchar_unlocked();
int x = 0;
while (c < '0' || c > '9')
{
c = getchar_unlocked();
}
while (c >= '0' && c <= '9')
{
x = (x << 1) + (x << 3) + c - '0';
c = getchar_unlocked();
}
return x;
};
void phongbeo();
const int inf = 1e9;
const int mod2 = 1e9+7;
const int mod1 = 998244353;
int add(int x,int y)
{
if(x+y>=mod2) x-=mod2;
if(x+y<0) x+=mod2;
return x+y;
}
struct icd
{
long double a;
int b;
};
struct ib
{
int a;
int b;
};
struct ic
{
int a, b, c, d;
};
struct ie
{
int a, b, c, d, e;
};
int n, m, s1, s2, s4, s3, sf, k, s5, s6, mx, s7, s8, s9, mx2, res, dem2 = 0, dem = 0, s33, dem3, dem4, mid, l2, r2, center;
int i, s10, s12,k1,k2,k3,s11,limit,w,l,r,last,root;
int kk,invf;
int el = 19;
main()
{
if(fopen(task".inp","r"))
{
fin(task);
fou(task);
}
NHP
/// cin>>s1;
///modwwe
phongbeo();
// checktime
}
int c[200001];
int a[200001];
set<int> s;
struct IT{
int t[800001];
void upd(int node,int l,int r,int l1)
{
if(l==r)
{
t[node]=1;
return;
}
int mid=l+r>>1;
if(l1<=mid)
upd(node<<1,l,mid,l1);
else upd(node<<1|1,mid+1,r,l1);
t[node]=t[node<<1]+t[node<<1|1];
}
int get(int node,int l,int r,int l1,int r1)
{
if(l>r1||r<l1||t[node]==0) return 0;
if(l>=l1&&r<=r1) return t[node];
int mid=l+r>>1;
return get(node<<1,l,mid,l1,r1)+get(node<<1|1,mid+1,r,l1,r1);
}
}st;
void phongbeo()
{
cin>>n;
for(int i=1;i<=n;i++)
cin>>a[i],s.insert(i);
for(int i=n;i>=1;--i)
{
auto it=s.upper_bound(a[i]);
if(it==s.begin()){cout<<-1; exit(0);}
it=prev(it);
c[*it]=i;
s.erase(it);
}
for(int i=1;i<=n;i++)
{
s4+=st.get(1,1,n,c[i],n)+c[i]-i;
st.upd(1,1,n,c[i]);
}
cout<<s4;
}
