#include <bits/stdc++.h>
#define DIM 100007
#define INF 10000000000007
#define fi first
#define se second

using namespace std;

typedef pair<long long, long long> pairll;
typedef long long ll;
typedef long double ld;

ll n,m,k,i,j,res,res1,sum,x,i1,a1,d,f,lasti,lasti1;
ll a[DIM],b[DIM];
pairll result[DIM];
string s1,s;
set<ll>::iterator it;
set< ll > s11;

int main()
{
    cin >> n;
    for(i=1; i<=n; i++){
        cin >> a[i];
        if(a[i]==1) k++;
        sum+=a[i];
    }
    if(sum%2==1) {
        cout << -1 << endl;
        return 0;
    }
    sort(a+1,a+n+1);
    for(i=1; i<=n; i++){
        if(a[i]==a[i+1]) {
            a[i]=0;
            a[i+1]=0;
            i++;
        }
        else if(a[i]%2==0){
            d++;
            result[d]={a[i],a[i]/2};
            a[i]=0;
        }
        else {
            x++;
            b[x]=a[i];
        }
    }
    for(i=2; i<=x; i+=2){
        d++;
        result[d]={b[i],b[i-1]};
    }
    cout << d << endl;
    for(i=1; i<=d; i++){
        cout << result[i].fi << " " << result[i].se << endl;
    }
    return 0;
}