import std;
import core.bitop;

// dfmt off
ulong MAX = 100_100, MOD = 1_000_000_007, INF = 1_000_000_000_000;
alias sread = () => readln.chomp();
alias lread(T = long) = () => readln.chomp.to!(T);
alias aryread(T = long) = () => readln.split.to!(T[]);
void aryWrite(T = long)(T[] ary){ ary.map!(x => x.text()).join(' ').writeln(); }
alias Pair = Tuple!(long, "H", long, "W", long, "cost");
alias PQueue(T, alias less = "a>b") = BinaryHeap!(Array!T, less);
// dfmt on

void main()
{
    auto n = lread();
    auto a = new long[](n);
    a.each!((ref x) => x = lread());
    auto c = new long[](n - 1);
    iota(n - 1).each!(i => c[i] = (a[i + 1] >= a[i] ? 1 : a[i] - a[i + 1]));
    writeln(a[0] > 0 ? -1 : c.sum());
}

void scan(TList...)(ref TList Args)
{
    auto line = readln.split();
    foreach (i, T; TList)
    {
        T val = line[i].to!(T);
        Args[i] = val;
    }
}
