import std.stdio, std.conv, std.functional, std.string; import std.algorithm, std.array, std.container, std.range, std.typecons; import std.bigint, std.numeric, std.math, std.random; import core.bitop; string FMT_F = "%.10f"; static File _f; void file_io(string fn) { _f = File(fn, "r"); } static string[] s_rd; T _RD(T = long)() { while(!s_rd.length) s_rd = readln.chomp.split; string res = s_rd[0]; s_rd.popFront; return res.to!T; } T _RD(T = long)(File f) { while(!s_rd.length) s_rd = f.readln.chomp.split; string res = s_rd[0]; s_rd.popFront; return res.to!T; } T[] _RDA(T = long)(T fix = 0) { auto r = readln.chomp.split.to!(T[]); r[] += fix; return r; } T[] _RDA(T = long)(File f, T fix = 0) { auto r = f.readln.chomp.split.to!(T[]); r[] += fix; return r; } T RD(T = long)() { if (_f.isOpen) return _RD!T(_f); else return _RD!T; } T[] RDA(T = long)(T fix = 0) { if (_f.isOpen) return _RDA!T(_f, fix); else return _RDA!T(fix); } size_t[] MAKE_IDX(alias less = "a < b", Range)(Range range) { auto idx = new size_t[](range.length); makeIndex!(less)(range, idx); return idx;} size_t MIN_POS(alias less = "a < b", Range)(Range range) { auto r = minPos!(less)(range); return range.length - r.length; } void chmin(T)(ref T x, T y) { x = min(x, y); } void chmax(T)(ref T x, T y) { x = max(x, y); } bool inside(T)(T x, T b, T e) { return x >= b && x < e; } T lcm(T)(T x, T y) { return x * (y / gcd(x, y)); } double[] rotate(double[] vec, double rad) { return [cos(rad)*vec[0] - sin(rad)*vec[1], sin(rad)*vec[0] + cos(rad)*vec[1]]; } long mod = 10^^9 + 7; //long mod = 998_244_353; //long mod = 1_000_003; void moda(ref long x, long y) { x = (x + y) % mod; } void mods(ref long x, long y) { x = ((x + mod) - (y % mod)) % mod; } void modm(ref long x, long y) { x = (x * y) % mod; } void modpow(ref long x, long y) { if (!y) { x = 1; return; } auto t = x; x.modpow(y>>1); x.modm(x); if (y&1) x.modm(t); } void modd(ref long x, long y) { y.modpow(mod - 2); x.modm(y); } void main() { auto N = RD; auto M = RD; auto edges = new long[][](N); auto par = new bool[long][](N); auto chi = new bool[long][](N); foreach (i; 0..M) { auto U = RD-1; auto V = RD-1; edges[U] ~= V; par[V][U] = true; chi[U][V] = true; } auto used = new bool[](N); foreach (i; 0..N) { if (par[i].empty && chi[i].empty) used[i] = true; } long[] l, r; void f(long i) { if (used[i]) return; if (par[i].empty) { used[i] = true; l ~= i; foreach (j; chi[i].keys) { par[j].remove(i); if (par[j].empty) f(j); } } else if (chi[i].empty) { used[i] = true; r ~= i; foreach (j; par[i].keys) { chi[j].remove(i); if (chi[j].empty) f(j); } } } foreach (i; 0..N) { f(i); debug writeln("i:", i, " ", l, r); debug writeln(par); debug writeln(chi); } foreach (i; 0..N) { f(i); } long[] ans; foreach (e; l) ans ~= e; foreach_reverse (e; r) ans ~= e; bool isLoop; foreach (i; 0..N) { if (!used[i]) { isLoop = true; ans ~= i; } } if (isLoop) writeln(ans.length); else writeln(ans.length-1); foreach (i; 0..ans.length-1) writeln(ans[i]+1, " ", ans[i+1]+1); if (isLoop) writeln(ans[$-1]+1, " ", ans[0]+1); stdout.flush; debug readln; }