Buggy python submission (verdict: Time Limit Exceeded):
import sys
sys.setrecursionlimit(10**6)
input = sys.stdin.readline
n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
visited_e = [False] * m
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append([b, i])
    edge[b].append([a, i])
 
ans = []
def dfs(prev, n):
    child_num = 0
    for v, index in edge[n]:
        if prev == v:
            continue
        if visited_e[index]:
            continue
        visited_e[index] = True
        if dfs(n, v) == 1:
            child_num += 1
            ans.append([n+1, v+1])
    if child_num % 2 == 0:
        return 1
    else:
        ans.append([n+1, prev+1])
        return 0
 
dfs(-1, 0)
print("\n".join([str(f) + " " + str(t) for f, t in ans]))

Fixed submission:
import sys
sys.setrecursionlimit(10**6)
input = sys.stdin.readline

n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
visited_e = [False] * m
visited_v = [False] * n
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append([b, i])
    edge[b].append([a, i])
 
ans = []
def dfs(prev, n):
    if visited_v[n]:
        return 1
    visited_v[n] = True
    child_num = 0
    for v, index in edge[n]:
        if visited_e[index]:
            continue
        visited_e[index] = True
        if dfs(n, v) == 1:
            child_num += 1
            ans.append([n+1, v+1])
    if child_num % 2 == 0:
        return 1
    else:
        ans.append([n+1, prev+1])
        return 0
 
dfs(-1, 0)
print("\n".join([str(f) + " " + str(t) for f, t in ans]))

Buggy python submission (verdict: Time Limit Exceeded):
import sys
sys.setrecursionlimit(10**6)
input = sys.stdin.readline
n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
vset = set()
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append(b)
    edge[b].append(a)
 
ans = []
def dfs(prev, n):
    if prev < n:
        key = str(prev) + "_" + str(n)
        if key in vset:
            return 0 
        else:
            vset.add(key)
    else:
        key = str(n) + "_" + str(prev)
        if key in vset:
            return 0 
        else:
            vset.add(key)
    child_num = 0
    for v in edge[n]:
        if prev == v:
            continue
        if dfs(n, v) == 1:
            child_num += 1
            ans.append([n+1, v+1])
    if child_num % 2 == 0:
        return 1
    else:
        ans.append([n+1, prev+1])
        return 0
 
dfs(-1, max(0, n - 2))
print("\n".join([str(f) + " " + str(t) for f, t in ans]))

Fixed submission:
import sys
sys.setrecursionlimit(10**6)
input = sys.stdin.readline

n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
visited_e = [False] * m
visited_v = [False] * n
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append([b, i])
    edge[b].append([a, i])
 
ans = []
def dfs(prev, n):
    if visited_v[n]:
        return 1
    visited_v[n] = True
    child_num = 0
    for v, index in edge[n]:
        if visited_e[index]:
            continue
        visited_e[index] = True
        if dfs(n, v) == 1:
            child_num += 1
            ans.append([n+1, v+1])
    if child_num % 2 == 0:
        return 1
    else:
        ans.append([n+1, prev+1])
        return 0
 
dfs(-1, 0)
print("\n".join([str(f) + " " + str(t) for f, t in ans]))

Buggy python submission (verdict: Time Limit Exceeded):
import sys
sys.setrecursionlimit(10**6)
input = sys.stdin.readline
n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
vset = set()
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append(b)
    edge[b].append(a)
 
ans = []
def dfs(prev, n):
    if prev < n:
        if (prev, n) in vset:
            return 0 
        else:
            vset.add((prev, n))
    else:
        if (n, prev) in vset:
            return 0 
        else:
            vset.add((n, prev))
    child_num = 0
    for v in edge[n]:
        if prev == v:
            continue
        if dfs(n, v) == 1:
            child_num += 1
            ans.append([n+1, v+1])
    if child_num % 2 == 0:
        return 1
    else:
        ans.append([n+1, prev+1])
        return 0
 
dfs(-1, max(0, n - 2))
print("\n".join([str(f) + " " + str(t) for f, t in ans]))

Fixed submission:
import sys
sys.setrecursionlimit(10**6)
input = sys.stdin.readline

n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
visited_e = [False] * m
visited_v = [False] * n
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append([b, i])
    edge[b].append([a, i])
 
ans = []
def dfs(prev, n):
    if visited_v[n]:
        return 1
    visited_v[n] = True
    child_num = 0
    for v, index in edge[n]:
        if visited_e[index]:
            continue
        visited_e[index] = True
        if dfs(n, v) == 1:
            child_num += 1
            ans.append([n+1, v+1])
    if child_num % 2 == 0:
        return 1
    else:
        ans.append([n+1, prev+1])
        return 0
 
dfs(-1, 0)
print("\n".join([str(f) + " " + str(t) for f, t in ans]))

Buggy python submission (verdict: Time Limit Exceeded):
import sys
sys.setrecursionlimit(10**6)
input = sys.stdin.readline
n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
vset = set()
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append(b)
    edge[b].append(a)

ans = []
def dfs(prev, n):
    if prev < n:
        if (prev, n) in vset:
            return 0 
        else:
            vset.add((prev, n))
    else:
        if (n, prev) in vset:
            return 0 
        else:
            vset.add((n, prev))
    child_num = 0
    for v in edge[n]:
        if prev == v:
            continue
        if dfs(n, v) == 1:
            child_num += 1
            ans.append([n+1, v+1])
    if child_num % 2 == 0:
        return 1
    else:
        ans.append([n+1, prev+1])
        return 0

dfs(-1, max(0, n - 2))
print("\n".join([str(f) + " " + str(t) for f, t in ans]))

Fixed submission:
import sys
sys.setrecursionlimit(10**6)
input = sys.stdin.readline

n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
visited_e = [False] * m
visited_v = [False] * n
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append([b, i])
    edge[b].append([a, i])
 
ans = []
def dfs(prev, n):
    if visited_v[n]:
        return 1
    visited_v[n] = True
    child_num = 0
    for v, index in edge[n]:
        if visited_e[index]:
            continue
        visited_e[index] = True
        if dfs(n, v) == 1:
            child_num += 1
            ans.append([n+1, v+1])
    if child_num % 2 == 0:
        return 1
    else:
        ans.append([n+1, prev+1])
        return 0
 
dfs(-1, 0)
print("\n".join([str(f) + " " + str(t) for f, t in ans]))

Buggy python submission (verdict: Time Limit Exceeded):
import sys
sys.setrecursionlimit(10**6)
input = sys.stdin.readline
n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
vmap = dict()
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append(b)
    edge[b].append(a)

def dfs(prev, n):
    if prev < n:
        if (prev, n) in vmap:
            return 0 
        else:
            vmap[(prev, n)] = 1
    else:
        if (n, prev) in vmap:
            return 0 
        else:
            vmap[(n, prev)] = 1
    child_num = 0
    for v in edge[n]:
        if prev == v:
            continue
        if dfs(n, v) == 1:
            child_num += 1
            print(n+1, v+1)
    if child_num % 2 == 0:
        return 1
    else:
        print(n+1, prev+1)
        return 0

dfs(-1, max(0, n - 2))

Fixed submission:
import sys
sys.setrecursionlimit(10**6)
input = sys.stdin.readline

n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
visited_e = [False] * m
visited_v = [False] * n
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append([b, i])
    edge[b].append([a, i])
 
ans = []
def dfs(prev, n):
    if visited_v[n]:
        return 1
    visited_v[n] = True
    child_num = 0
    for v, index in edge[n]:
        if visited_e[index]:
            continue
        visited_e[index] = True
        if dfs(n, v) == 1:
            child_num += 1
            ans.append([n+1, v+1])
    if child_num % 2 == 0:
        return 1
    else:
        ans.append([n+1, prev+1])
        return 0
 
dfs(-1, 0)
print("\n".join([str(f) + " " + str(t) for f, t in ans]))

Buggy python submission (verdict: Time Limit Exceeded):
import sys
sys.setrecursionlimit(10**6)
input = sys.stdin.readline
n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
vmap = dict()
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append(b)
    edge[b].append(a)
 
def dfs(prev, n):
    if prev < n:
        if (prev, n) in vmap:
            return 0 
        else:
            vmap[(prev, n)] = 1
    else:
        if (n, prev) in vmap:
            return 0 
        else:
            vmap[(n, prev)] = 1
    child_num = 0
    for v in edge[n]:
        if prev == v:
            continue
        if dfs(n, v) == 1:
            child_num += 1
            print(n+1, v+1)
    if child_num % 2 == 0:
        return 1
    else:
        print(n+1, prev+1)
        return 0

dfs(-1, (n - 1) // 2)

Fixed submission:
import sys
sys.setrecursionlimit(10**6)
input = sys.stdin.readline

n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
visited_e = [False] * m
visited_v = [False] * n
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append([b, i])
    edge[b].append([a, i])
 
ans = []
def dfs(prev, n):
    if visited_v[n]:
        return 1
    visited_v[n] = True
    child_num = 0
    for v, index in edge[n]:
        if visited_e[index]:
            continue
        visited_e[index] = True
        if dfs(n, v) == 1:
            child_num += 1
            ans.append([n+1, v+1])
    if child_num % 2 == 0:
        return 1
    else:
        ans.append([n+1, prev+1])
        return 0
 
dfs(-1, 0)
print("\n".join([str(f) + " " + str(t) for f, t in ans]))

Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque
import sys
sys.setrecursionlimit(10**6)
input = sys.stdin.readline
n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
vmap = dict()
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append(b)
    edge[b].append(a)

dq = deque()
dq.append([-1, 0])
def dfs(prev, n):
    if prev < n:
        if (prev, n) in vmap:
            return 0 
        else:
            vmap[(prev, n)] = 1
    else:
        if (n, prev) in vmap:
            return 0 
        else:
            vmap[(n, prev)] = 1
    child_num = 0
    for v in edge[n]:
        if prev == v:
            continue
        if dfs(n, v) == 1:
            child_num += 1
            print(n+1, v+1)
    if child_num % 2 == 0:
        return 1
    else:
        print(n+1, prev+1)
        return 0

dfs(-1, 0)

Fixed submission:
import sys
sys.setrecursionlimit(10**6)
input = sys.stdin.readline

n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
visited_e = [False] * m
visited_v = [False] * n
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append([b, i])
    edge[b].append([a, i])
 
ans = []
def dfs(prev, n):
    if visited_v[n]:
        return 1
    visited_v[n] = True
    child_num = 0
    for v, index in edge[n]:
        if visited_e[index]:
            continue
        visited_e[index] = True
        if dfs(n, v) == 1:
            child_num += 1
            ans.append([n+1, v+1])
    if child_num % 2 == 0:
        return 1
    else:
        ans.append([n+1, prev+1])
        return 0
 
dfs(-1, 0)
print("\n".join([str(f) + " " + str(t) for f, t in ans]))

Buggy python submission (verdict: Runtime Error):
input = sys.stdin.readline
n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
vmap = dict()
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append(b)
    edge[b].append(a)
 
def dfs(prev, n):
    if prev < n:
        if (prev, n) in vmap:
            return 0 
        else:
            vmap[(prev, n)] = 1
    else:
        if (n, prev) in vmap:
            return 0 
        else:
            vmap[(n, prev)] = 1
    child_num = 0
    for v in edge[n]:
        if prev == v:
            continue
        if dfs(n, v) == 1:
            child_num += 1
            print(n+1, v+1)
    if child_num % 2 == 0:
        return 1
    else:
        print(n+1, prev+1)
        return 0
 
dfs(-1, n // 2)

Fixed submission:
import sys
sys.setrecursionlimit(10**6)
input = sys.stdin.readline

n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
visited_e = [False] * m
visited_v = [False] * n
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append([b, i])
    edge[b].append([a, i])
 
ans = []
def dfs(prev, n):
    if visited_v[n]:
        return 1
    visited_v[n] = True
    child_num = 0
    for v, index in edge[n]:
        if visited_e[index]:
            continue
        visited_e[index] = True
        if dfs(n, v) == 1:
            child_num += 1
            ans.append([n+1, v+1])
    if child_num % 2 == 0:
        return 1
    else:
        ans.append([n+1, prev+1])
        return 0
 
dfs(-1, 0)
print("\n".join([str(f) + " " + str(t) for f, t in ans]))

Buggy python submission (verdict: Time Limit Exceeded):
import sys
sys.setrecursionlimit(10**6)
input = sys.stdin.readline
n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
vmap = dict()
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append(b)
    edge[b].append(a)

def dfs(prev, n):
    if prev < n:
        if (prev, n) in vmap:
            return 0 
        else:
            vmap[(prev, n)] = 1
    else:
        if (n, prev) in vmap:
            return 0 
        else:
            vmap[(n, prev)] = 1
    child_num = 0
    for v in edge[n]:
        if prev == v:
            continue
        if dfs(n, v) == 1:
            child_num += 1
            print(n+1, v+1)
    if child_num % 2 == 0:
        return 1
    else:
        print(n+1, prev+1)
        return 0

dfs(-1, 0)

Fixed submission:
import sys
sys.setrecursionlimit(10**6)
input = sys.stdin.readline

n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
visited_e = [False] * m
visited_v = [False] * n
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append([b, i])
    edge[b].append([a, i])
 
ans = []
def dfs(prev, n):
    if visited_v[n]:
        return 1
    visited_v[n] = True
    child_num = 0
    for v, index in edge[n]:
        if visited_e[index]:
            continue
        visited_e[index] = True
        if dfs(n, v) == 1:
            child_num += 1
            ans.append([n+1, v+1])
    if child_num % 2 == 0:
        return 1
    else:
        ans.append([n+1, prev+1])
        return 0
 
dfs(-1, 0)
print("\n".join([str(f) + " " + str(t) for f, t in ans]))

Buggy python submission (verdict: Time Limit Exceeded):
import sys
sys.setrecursionlimit(10**6)
input = sys.stdin.readline
n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
vmap = dict()
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append(b)
    edge[b].append(a)

def dfs(prev, n):
    if prev < n:
        if (prev, n) in vmap:
            return 0 
        else:
            vmap[(prev, n)] = 1
    else:
        if (n, prev) in vmap:
            return 0 
        else:
            vmap[(n, prev)] = 1
    child_num = 0
    for v in edge[n]:
        if prev == v:
            continue
        if dfs(n, v) == 1:
            child_num += 1
            print(n+1, v+1)
    if child_num % 2 == 0:
        return 1
    else:
        print(n+1, prev+1)
        return 0

dfs(-1, 0)

Fixed submission:
import sys
sys.setrecursionlimit(10**6)
input = sys.stdin.readline

n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
visited_e = [False] * m
visited_v = [False] * n
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append([b, i])
    edge[b].append([a, i])
 
ans = []
def dfs(prev, n):
    if visited_v[n]:
        return 1
    visited_v[n] = True
    child_num = 0
    for v, index in edge[n]:
        if visited_e[index]:
            continue
        visited_e[index] = True
        if dfs(n, v) == 1:
            child_num += 1
            ans.append([n+1, v+1])
    if child_num % 2 == 0:
        return 1
    else:
        ans.append([n+1, prev+1])
        return 0
 
dfs(-1, 0)
print("\n".join([str(f) + " " + str(t) for f, t in ans]))

Buggy python submission (verdict: Time Limit Exceeded):
import sys
sys.setrecursionlimit(10**6)
input = sys.stdin.readline
n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
visited = [0] * n
vmap = dict()
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append(b)
    edge[b].append(a)

ans = []
def dfs(prev, n):
    if prev < n:
        if (prev, n) in vmap:
            return 0 
        else:
            vmap[(prev, n)] = 1
    else:
        if (n, prev) in vmap:
            return 0 
        else:
            vmap[(n, prev)] = 1
    child_num = 0
    for v in edge[n]:
        if prev == v:
            continue
        if dfs(n, v) == 1:
            child_num += 1
            ans.append([n, v])
    if child_num % 2 == 0:
        return 1
    else:
        ans.append([n, prev])
        return 0

dfs(-1, 0)
for f, t in ans:
    print(f+1, t+1)

Fixed submission:
import sys
sys.setrecursionlimit(10**6)
input = sys.stdin.readline

n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
visited_e = [False] * m
visited_v = [False] * n
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append([b, i])
    edge[b].append([a, i])
 
ans = []
def dfs(prev, n):
    if visited_v[n]:
        return 1
    visited_v[n] = True
    child_num = 0
    for v, index in edge[n]:
        if visited_e[index]:
            continue
        visited_e[index] = True
        if dfs(n, v) == 1:
            child_num += 1
            ans.append([n+1, v+1])
    if child_num % 2 == 0:
        return 1
    else:
        ans.append([n+1, prev+1])
        return 0
 
dfs(-1, 0)
print("\n".join([str(f) + " " + str(t) for f, t in ans]))

Buggy python submission (verdict: Time Limit Exceeded):
import sys
sys.setrecursionlimit(10**6)
n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
visited = [0] * n
vmap = dict()
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append(b)
    edge[b].append(a)

ans = []
def dfs(prev, n):
    if prev < n:
        if (prev, n) in vmap:
            return 0 
        else:
            vmap[(prev, n)] = 1
    else:
        if (n, prev) in vmap:
            return 0 
        else:
            vmap[(n, prev)] = 1
    child_num = 0
    for v in edge[n]:
        if prev == v:
            continue
        if dfs(n, v) == 1:
            child_num += 1
            ans.append([n, v])
    if child_num % 2 == 0:
        return 1
    else:
        ans.append([n, prev])
        return 0

dfs(-1, 0)
for f, t in ans:
    print(f+1, t+1)

Fixed submission:
import sys
sys.setrecursionlimit(10**6)
input = sys.stdin.readline

n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
visited_e = [False] * m
visited_v = [False] * n
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append([b, i])
    edge[b].append([a, i])
 
ans = []
def dfs(prev, n):
    if visited_v[n]:
        return 1
    visited_v[n] = True
    child_num = 0
    for v, index in edge[n]:
        if visited_e[index]:
            continue
        visited_e[index] = True
        if dfs(n, v) == 1:
            child_num += 1
            ans.append([n+1, v+1])
    if child_num % 2 == 0:
        return 1
    else:
        ans.append([n+1, prev+1])
        return 0
 
dfs(-1, 0)
print("\n".join([str(f) + " " + str(t) for f, t in ans]))

Buggy python submission (verdict: Time Limit Exceeded):
import sys
sys.setrecursionlimit(10**6)
n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
visited = [0] * n
vmap = dict()
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append(b)
    edge[b].append(a)

ans = []
def dfs(prev, n):
    if prev < n:
        if (prev, n) in vmap:
            return 0 
        else:
            vmap[(prev, n)] = 1
    else:
        if (n, prev) in vmap:
            return 0 
        else:
            vmap[(n, prev)] = 1
    child_num = 0
    for v in edge[n]:
        if prev == v:
            continue
        if dfs(n, v) == 1:
            child_num += 1
            ans.append([n, v])
    if child_num % 2 == 0:
        return 1
    else:
        ans.append([n, prev])
        return 0

dfs(-1, 0)
for f, t in ans:
    print(f+1, t+1)

Fixed submission:
import sys
sys.setrecursionlimit(10**6)
input = sys.stdin.readline

n, m = [int(item) for item in input().split()]
if m % 2 != 0:
    print(-1)
    exit()
visited_e = [False] * m
visited_v = [False] * n
edge = [[] for _ in range(n)]
for i in range(m):
    a, b = [int(item) for item in input().split()]
    a -= 1; b -= 1
    edge[a].append([b, i])
    edge[b].append([a, i])
 
ans = []
def dfs(prev, n):
    if visited_v[n]:
        return 1
    visited_v[n] = True
    child_num = 0
    for v, index in edge[n]:
        if visited_e[index]:
            continue
        visited_e[index] = True
        if dfs(n, v) == 1:
            child_num += 1
            ans.append([n+1, v+1])
    if child_num % 2 == 0:
        return 1
    else:
        ans.append([n+1, prev+1])
        return 0
 
dfs(-1, 0)
print("\n".join([str(f) + " " + str(t) for f, t in ans]))