import std.experimental.all;

T read(T)() { return readln.chomp.to!T; }
T[] reads(T)() { return readln.split.to!(T[]); }
alias readint = read!int;
alias readints = reads!int;

alias MatchResult = Tuple!(int, "count", int[], "left", int[], "right");
MatchResult maxBipartiteMatching(bool[][] g) {

    bool rec(int p, int[] left, int[] right, bool[][] g, bool[] used) {
        for (int i = 0; i < right.length; i++) {
            if (g[p][i] && right[i] == -1) {
                left[p] = i;
                right[i] = p;
                return true;
            }
        }

        for (int i = 0; i < right.length; i++) {
            if (g[p][i] && !used[i]) {
                used[i] = true;
                if (rec(right[i], left, right, g, used)) {
                    left[p] = i;
                    right[i] = p;
                    return true;
                }
            }
        }
        return false;
    }

    auto left = new int[g.length];
    auto right = new int[g[0].length];
    left[] = -1;
    right[] = -1;
    int cnt = 0;
    for (int i = 0; i < left.length; i++) {
        auto used = new bool[left.length];
        if (rec(i, left, right, g, used)) {
            cnt++;
        }
    }
    return MatchResult(cnt, left, right);
}

void main() {
    int n = readint;
    int[] a;
    for (int i = 0; i < n; i++) {
        a ~= readint;
    }

    auto g = new bool[][](n, n);
    for (int i = 0; i < n; i++) {
        g[i][] = true;
        g[i][a[i]] = false;
    }

    auto result = maxBipartiteMatching(g);
    if (result.count == result.left.length) {
        foreach (x; result.left)
            writeln(x);
    }
    else writeln(-1);
}