import sys

# sys.setrecursionlimit(200005)
# sys.set_int_max_str_digits(200005)
int1 = lambda x: int(x)-1
pDB = lambda *x: print(*x, end="\n", file=sys.stderr)
p2D = lambda x: print(*x, sep="\n", end="\n\n", file=sys.stderr)
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()

dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
# inf = -1-(-1 << 31)
inf = -1-(-1 << 62)

# md = 10**9+7
md = 998244353

n,m,s,g=LI()
s-=1
g-=1
to=[[] for _ in range(n)]
uv=[]
for _ in range(m):
    u,v=LI1()
    to[u].append(v)
    to[v].append(u)
    uv.append((u,v))

def dfs(root=0):
    uu = [root]
    dfsOrder[root] = 0
    lowlink[root] = 0
    eulerTour.append(root)
    # !!! to's format
    stack = [(root, v) for v in to[root]]
    while stack:
        u, v = stack.pop()
        if v == -1:
            p = parent[u]
            if lowlink[u] < lowlink[p]: lowlink[p] = lowlink[u]
            eulerTour.append(p)
            right[u] = len(uu)
        elif dfsOrder[v] == -1:
            parent[v] = u
            depth[v] = depth[u]+1
            dfsOrder[v] = len(uu)
            uu.append(v)
            stack.append((v, -1))
            # !!! to's format
            for nv in to[v]:
                if nv == u: continue
                stack.append((v, nv))
            lowlink[v] = dfsOrder[v]
            eulerTour.append(v)
            dfsTree[u].append(v)
        else:
            if dfsOrder[v] < lowlink[u]: lowlink[u] = dfsOrder[v]
    right[root] = len(uu)
    return uu

# vの親がpのときlow[v]>ord[p]だと橋
lowlink = [n]*n
eulerTour = []
dfsTree = [[] for _ in range(n)]
right = [-1]*n  # left = dfsOrder
parent, depth, dfsOrder = [-1]*n, [0]*n, [-1]*n

dfs()
to=[[] for _ in range(n)]
for u,v in uv:
    c=n-1
    if lowlink[u]>dfsOrder[v] or lowlink[v]>dfsOrder[u]:c=n
    to[u].append((v,c))
    to[v].append((u,c))

from heapq import *

def dijkstra(to, root=0):
    n = len(to)
    dist = [inf]*n
    dist[root] = 0
    hp = [(0, root)]
    while hp:
        d, u = heappop(hp)
        if d > dist[u]: continue
        for v, c in to[u]:
            nd = d+c
            if dist[v] <= nd: continue
            dist[v] = nd
            heappush(hp, (nd, v))

    return dist

dist=dijkstra(to,s)
d=dist[g]
d%=n
print(-d%n)