//不等,不問,不猶豫,不回頭.
#include
#define _ 0
#define ls p<<1
#define db double
#define rs p<<1|1
#define P 1000000007
#define ll long long
#define INF 1000000000
#define get(x) x=read()
#define PLI pair
#define PII pair
#define ull unsigned long long
#define put(x) printf("%d\n",x)
#define putl(x) printf("%lld\n",x)
#define rep(x,y,z) for(int x=y;x<=z;++x)
#define fep(x,y,z) for(int x=y;x>=z;--x)
#define go(x) for(int i=link[x],y=a[i].y;i;y=a[i=a[i].next].y)
using namespace std;
const int N=1e5+10;
int n,m1,m2,f[3][N],vis[N],num,bx[N],by[N],id[N];
struct wy{int len,v;}a[N];
vectorv[N];
mapmp;
sets;
inline int read()

{

int x=0,ff=1;

char ch=getchar();

while(!isdigit(ch)) {if(ch=='-') ff=-1;ch=getchar();}

while(isdigit(ch)) {x=(x<<1)+(x<<3)+(ch^48);ch=getchar();}

return x*ff;

}
 

inline int getf(int op,int x){return f[op][x]==x?x:f[op][x]=getf(op,f[op][x]);}
inline void init()

{

get(n);get(m1);get(m2);

rep(i,1,n) f[1][i]=f[2][i]=i;

rep(i,1,m1)

{

int get(x),get(y);

int t1=getf(1,x),t2=getf(1,y);

if(t1!=t2) f[1][t1]=t2;

}

rep(i,1,m2)

{

int get(x),get(y);

int t1=getf(2,x),t2=getf(2,y);

if(t1!=t2) f[2][t1]=t2;

}

}
 

inline bool cmp(wy a,wy b) {return a.len>b.len;}
inline void prework()

{

rep(i,1,n) f[1][i]=getf(1,i),f[2][i]=getf(2,i);

rep(i,1,n)

{

if(!vis[f[1][i]])

{

vis[f[1][i]]=1;

id[f[1][i]]=++num;

}

a[id[f[1][i]]].len++;

a[id[f[1][i]]].v=f[1][i];

}

sort(a+1,a+num+1,cmp);

rep(i,1,num) id[a[i].v]=i;

rep(i,1,n) v[id[f[1][i]]].push_back(i);

memset(vis,0,sizeof(vis));

for(auto x:v[1])

{

if(vis[f[2][x]]) continue;

vis[f[2][x]]=1;

s.insert(f[2][x]);

mp[f[2][x]]=x;

}

}
inline void solve()

{

int ans=0;

rep(i,2,num)//列舉從第二個連通塊開始的所有連通塊，嘗試與第一個連通塊合併。

{

bool flag=false;

for(auto x:s)//列舉每個連通塊

{

if(flag) break;

for(auto y:v[i])

{

int t=getf(2,y);

if(t!=x&&!flag)

{

bx[++ans]=mp[x];

by[ans]=y;

f[2][t]=x;

if(s.find(t)!=s.end()) s.erase(t);

flag=true;

}

else if(flag&&!vis[t])

{

vis[t]=1;

s.insert(t);

mp[t]=y;

}

}

}

}

put(ans);

rep(i,1,ans) printf("%d %d\n",bx[i],by[i]);

}
int main()

{	

// freopen("1.in","r",stdin);

//freopen("sol.out","w",stdout);

init();

prework();

solve();

return (0_0);

}

//以吾之血,鑄吾最後的亡魂.