Buggy python submission (verdict: Time Limit Exceeded):
import math, string, itertools, fractions, heapq, collections, re,  array, bisect, sys, random, time, copy, functools
sys.setrecursionlimit(10**7)
inf = 10 ** 20
eps = 1.0 / 10**10
mod = 10**9+7
dd = [(-1, 0), (0, 1), (1, 0), (0, -1)]
ddn = [(-1, 0), (-1, 1), (0, 1), (1, 1), (1, 0), (1, -1), (0, -1), (-1, -1)]
def LI(): return [int(x) for x in sys.stdin.readline().split()]
def LI_(): return [int(x)-1 for x in sys.stdin.readline().split()]
def LF(): return [float(x) for x in sys.stdin.readline().split()]
def LS(): return sys.stdin.readline().split()
def _I(): return int(sys.stdin.readline())
def _F(): return float(sys.stdin.readline())
def _pf(s): return print(s, flush=True)


N, M = LI()
LR = []

for i in range(M):
    LR.append(LI())

#低い方と高い方を求める
lower = None
found = False
for i in range(1,N+1):
    if found:
        break
    for lr in LR:
        if not (lr[0] <= i <= lr[1]):
            break
    else:
        lower = i
        found = True

if lower == None:
    print(0)
    exit()
#  print('lower', lower)
if lower == N:
    print(1)
    exit()

upper = None
found = False
for i in reversed(range(lower+1, N+1)):
    if found:
        break
    for lr in LR:
        if not (lr[0] <= i <= lr[1]):
            break
    else:
        upper = i
        found = True
if upper == None:
    upper = lower
#  print('upper', upper)
print(upper - lower + 1)




Fixed submission:
import math, string, itertools, fractions, heapq, collections, re,  array, bisect, sys, random, time, copy, functools
sys.setrecursionlimit(10**7)
inf = 10 ** 20
eps = 1.0 / 10**10
mod = 10**9+7
dd = [(-1, 0), (0, 1), (1, 0), (0, -1)]
ddn = [(-1, 0), (-1, 1), (0, 1), (1, 1), (1, 0), (1, -1), (0, -1), (-1, -1)]
def LI(): return [int(x) for x in sys.stdin.readline().split()]
def LI_(): return [int(x)-1 for x in sys.stdin.readline().split()]
def LF(): return [float(x) for x in sys.stdin.readline().split()]
def LS(): return sys.stdin.readline().split()
def _I(): return int(sys.stdin.readline())
def _F(): return float(sys.stdin.readline())
def _pf(s): return print(s, flush=True)


N, M = LI()
LR = []

for i in range(M):
    LR.append(LI())

left, right = 0, inf
for lr in LR:
    left = max(left, lr[0])
    right = min(right, lr[1])
#  print(left, right)
if left > right:
    print(0)
else:
    print(right - left + 1)

Buggy python submission (verdict: Runtime Error):
import math, string, itertools, fractions, heapq, collections, re,  array, bisect, sys, random, time, copy, functools
sys.setrecursionlimit(10**7)
inf = 10 ** 20
eps = 1.0 / 10**10
mod = 10**9+7
dd = [(-1, 0), (0, 1), (1, 0), (0, -1)] ddn = [(-1, 0), (-1, 1), (0, 1), (1, 1), (1, 0), (1, -1), (0, -1), (-1, -1)]
def LI(): return [int(x) for x in sys.stdin.readline().split()]
def LI_(): return [int(x)-1 for x in sys.stdin.readline().split()]
def LF(): return [float(x) for x in sys.stdin.readline().split()]
def LS(): return sys.stdin.readline().split()
def _I(): return int(sys.stdin.readline())
def _F(): return float(sys.stdin.readline())
def _pf(s): return print(s, flush=True)


N, M = LI()
LR = []

for i in range(M):
    LR.append(LI())

#低い方と高い方を求める
lower = None
found = False
for i in range(1,N+1):
    if found:
        break
    for lr in LR:
        if not (lr[0] <= i <= lr[1]):
            break
    else:
        lower = i
        found = True

if lower == None:
    print(0)
    exit()
#  print('lower', lower)
if lower == N:
    print(1)
    exit()

upper = None
found = False
for i in reversed(range(lower+1, N+1)):
    if found:
        break
    for lr in LR:
        if not (lr[0] <= i <= lr[1]):
            break
    else:
        upper = i
        found = True
if upper == None:
    upper = lower
#  print('upper', upper)
print(upper - lower + 1)




Fixed submission:
import math, string, itertools, fractions, heapq, collections, re,  array, bisect, sys, random, time, copy, functools
sys.setrecursionlimit(10**7)
inf = 10 ** 20
eps = 1.0 / 10**10
mod = 10**9+7
dd = [(-1, 0), (0, 1), (1, 0), (0, -1)]
ddn = [(-1, 0), (-1, 1), (0, 1), (1, 1), (1, 0), (1, -1), (0, -1), (-1, -1)]
def LI(): return [int(x) for x in sys.stdin.readline().split()]
def LI_(): return [int(x)-1 for x in sys.stdin.readline().split()]
def LF(): return [float(x) for x in sys.stdin.readline().split()]
def LS(): return sys.stdin.readline().split()
def _I(): return int(sys.stdin.readline())
def _F(): return float(sys.stdin.readline())
def _pf(s): return print(s, flush=True)


N, M = LI()
LR = []

for i in range(M):
    LR.append(LI())

left, right = 0, inf
for lr in LR:
    left = max(left, lr[0])
    right = min(right, lr[1])
#  print(left, right)
if left > right:
    print(0)
else:
    print(right - left + 1)

Buggy python submission (verdict: Runtime Error):
import math, string, itertools, fractions, heapq, collections, re,  array, bisect, sys, random, time, copy, functools
sys.setrecursionlimit(10**7)
inf = 10 ** 20
eps = 1.0 / 10**10
mod = 10**9+7
dd = [(-1, 0), (0, 1), (1, 0), (0, -1)]
ddn = [(-1, 0), (-1, 1), (0, 1), (1, 1), (1, 0), (1, -1), (0, -1), (-1, -1)]
def LI(): return [int(x) for x in sys.stdin.readline().split()]
def LI_(): return [int(x)-1 for x in sys.stdin.readline().split()]
def LF(): return [float(x) for x in sys.stdin.readline().split()]
def LS(): return sys.stdin.readline().split()
def _I(): return int(sys.stdin.readline())
def _F(): return float(sys.stdin.readline())
def _pf(s): return print(s, flush=True)


N, M = LI()
LR = []

for i in range(M):
    LR.append(LI())

#低い方と高い方を求める
lower = None
found = False
for i in range(1,N+1):
    if found:
        break
    for lr in LR:
        if not (lr[0] <= i <= lr[1]):
            break
    else:
        lower = i
        found = True

#  print('lower', lower)
if lower == N:
    print(1)
    exit()

upper = None
found = False
for i in reversed(range(lower+1, N+1)):
    if found:
        break
    for lr in LR:
        if not (lr[0] <= i <= lr[1]):
            break
    else:
        upper = i
        found = True
if upper == None:
    upper = lower
#  print('upper', upper)
print(upper - lower + 1)




Fixed submission:
import math, string, itertools, fractions, heapq, collections, re,  array, bisect, sys, random, time, copy, functools
sys.setrecursionlimit(10**7)
inf = 10 ** 20
eps = 1.0 / 10**10
mod = 10**9+7
dd = [(-1, 0), (0, 1), (1, 0), (0, -1)]
ddn = [(-1, 0), (-1, 1), (0, 1), (1, 1), (1, 0), (1, -1), (0, -1), (-1, -1)]
def LI(): return [int(x) for x in sys.stdin.readline().split()]
def LI_(): return [int(x)-1 for x in sys.stdin.readline().split()]
def LF(): return [float(x) for x in sys.stdin.readline().split()]
def LS(): return sys.stdin.readline().split()
def _I(): return int(sys.stdin.readline())
def _F(): return float(sys.stdin.readline())
def _pf(s): return print(s, flush=True)


N, M = LI()
LR = []

for i in range(M):
    LR.append(LI())

left, right = 0, inf
for lr in LR:
    left = max(left, lr[0])
    right = min(right, lr[1])
#  print(left, right)
if left > right:
    print(0)
else:
    print(right - left + 1)