#include<bits/stdc++.h>
using namespace std;
#define int long long
#define PII pair<int,int>
#define lson 2*p
#define rson 2*p+1
#define x first
#define y second
#define LL long long
#define lowbit(x) x&-x
#define endl "\n"
const int N=1e6+10;
const int mod=1e9+7;
int a[N],cnt[N];
void solve() {
	int n;
	cin>>n;
	priority_queue<PII> q;
	int sum=0;
	set<int> pos;
	set<PII> st;
	vector<PII> ans;
	int maxn=0;
	for(int i=1;i<=n;i++) {
		cin>>a[i];
		sum+=a[i];
		maxn=max(maxn,a[i]);
		cnt[a[i]]++;
		if(!a[i]) continue;;
		pos.insert(i);
		st.insert({a[i],-i});
	}
	if(sum&1) {
		if(maxn*2>sum) {
			for(int i=1;i<=n;i++) {
				if(a[i]==maxn) {
					ans.push_back({i,i});
					cnt[maxn]--;
					cnt[maxn-1]++;
					a[i]--;
					if(!a[i]) pos.erase(i);
					while(!cnt[maxn]) maxn--;
					break;
				}
			}
		} else {
			for(int i=1;i<=n;i++) {
				if(!a[i]) continue;
				ans.push_back({i,i});
				cnt[a[i]]--;
				cnt[a[i]-1]++;
				a[i]--;
				if(!a[i]) pos.erase(i);
				while(!cnt[maxn]) maxn--;
				break;
			}
		}
		sum--;
	}
	int flag=0;
	while(maxn*2<sum) {
		flag=1;
		int x=*pos.begin();
		pos.erase(x);
		int y=*pos.begin();
		pos.erase(y);
 		cnt[a[x]]--,cnt[a[y]]--;
		st.erase({a[x],-x});
		st.erase({a[y],-y});
		a[x]--,a[y]--;
		if(a[x]) st.insert({a[x],-x});
		if(a[y]) st.insert({a[y],-y});
		cnt[a[x]]++,cnt[a[y]]++;
		ans.push_back({x,y});
		while(!cnt[maxn]) maxn--;
		sum-=2;
		if(a[x]) pos.insert(x);
		if(a[y]) pos.insert(y);
	}
	while(maxn*2>sum) {
		flag=1;
		auto [x,y]=*st.rbegin();
		ans.push_back({-y,-y});
		y=-y;
		st.erase({a[y],-y});
		cnt[a[y]]--;
		cnt[a[y]-1]++;
		a[y]--;
		sum--;
		while(!cnt[maxn]) maxn--;
		if(a[y]) st.insert({a[y],-y});
		if(!a[y]) pos.erase(y);
	}
	if(!flag) cout<<"OK"<<endl;
	auto [x,y]=*st.rbegin();
	y=-y;
	pos.erase(y);
	while(pos.size()) {
		if(!pos.size()) break;
		int x=*pos.begin();
		ans.push_back({min(x,y),max(x,y)});
		a[x]--,a[y]--;
		if(!a[x]) pos.erase(x);
	}
	sort(ans.begin(),ans.end());
	cout<<ans.size()<<endl;
	for(auto [u,v]:ans) cout<<u<<" "<<v<<endl;
}
signed main() {
    int T=1;
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    // cin>>T;
    while(T--) {    
        solve();
    }
}