class Graph(object):
    def __init__(self, n, N=100010):
        self.h = [-1] * N
        self.e = [-1] * N
        self.w = [-1] * N
        self.ne = [-1] * N
        self.idx = 0
        self.n = n

    def add(self,a,b,c):
        idx = self.idx
        self.e[idx] = b
        self.w[idx] = c
        self.ne[idx] = self.h[a]
        self.h[a] = idx 
        self.idx += 1

    def spfa(self,s,u):
        from queue import Queue        
        q = Queue()
        p = [float('inf')] * (self.n+1)
        st = [False] * (self.n+1)
        p[s] = 0
        st[s] = True
        q.put(s)
        while not q.empty():
            d = q.get()
            st[d] = False
            i = self.h[d]
            while i!= -1:
                v = self.e[i]
                w = self.w[i]
                if p[v] > p[d] + w:
                    p[v] = p[d] + w
                    if not st[v]:
                        q.put(v)
                        st[v] = True
                i = self.ne[i]
        return 'impossible' if p[u] == float('inf') else p[u]

n,m = map(int, input().split(' '))
g = Graph(n)
for _ in range(m):
  a, b, c = map(int, input().split(' ')) 
  g.add(a,b,c)
print(g.spfa(1,n))