ecnu 1817 Shortest path

class shortestpath():
    def __init__(self,x):
        self.n=x
        self.dis=[[float(‘inf‘) for i in range(0,x+1)] for j in range(0,x+1)]
        for i in range(0,x+1):
            self.dis[i][i]=0

    def dijkstra(self,s):
        lowdis=[float(‘inf‘)]*(self.n+1)
        vis=[False]*(self.n+1)
        vis[s]=True
        for i in range(0,self.n+1):
            lowdis[i]=self.dis[s][i]
        
        for i in range(1,self.n):
            mind=float(‘inf‘)
            minn=-1
            for j in range(0,self.n+1):
                if not vis[j]:
                    if lowdis[j]<mind:
                        mind=lowdis[j]
                        minn=j
            if mind<float(‘inf‘):
                vis[minn]=True
                for j in range(0,self.n+1):
                    if not vis[j]:
                        if mind+self.dis[minn][j]<lowdis[j]:
                            lowdis[j]=mind+self.dis[minn][j]
            else: return lowdis
        return lowdis

n,m=map(int,input().split())
d=shortestpath(n)
for i in range(0,m):
    u,v,w=map(int,input().split())
    if d.dis[u][v]>w:
        d.dis[u][v]=w

lowdis=d.dijkstra(1)
if lowdis[n]!=float(‘inf‘):
    print(lowdis[n])
else: print("-1")