#include<fstream>
#include<algorithm>
#include<vector>
using namespace std;
 
int i,j,n,m,k,aux,nr=0;
int ok,x,y;
bool v[101][101];
struct harta
{
    int out,in,poz;
};
harta a[101],b[101];
bool cmp(harta a,harta b)
{
    if(a.out==b.out)
    return a.poz<b.poz;
    return a.out>b.out;
}
bool cmp1(harta a,harta b)
{
    if(a.in==b.in)
    return a.out>b.out;
    return a.in>b.in;
}
int main()
{
    ifstream f("harta.in");
    ofstream g("harta.out");
    f>>n;
    for(i=1;i<=n;++i)
    {
    f>>a[i].out>>a[i].in;
    nr+=a[i].out;
    a[i].poz=i;
    b[i]=a[i];
    }
	g<<nr<<"\n";
    sort(a+1,a+n+1,cmp);
    for(i=1;i<=n;++i)
    {
        sort(b+1,b+n+1,cmp1);
	    j=1;
        while(a[i].out)
        {
            x=a[i].poz,y=b[j].poz;
			if(x!=y)
			{
			v[x][y]=1;
        --a[i].out;
        --b[j].in;
			}
			++j;
        }
    }
	for(i=1;i<=n;++i)
		for(j=1;j<=n;++j)
			if(v[i][j])
				g<<i<<" "<<j<<"\n";
    return 0;
}