from heapq import  heappop,heappush
class Heap_Point:
        def __init__(self,x,d):
            self.d=d
            self.x=x

        def __str__(self):
            return "(point:{}, dist:{})".format(self.x,self.d)

        def __repr__(self):
            return str(self)

        def __lt__(self,other):
            return self.d<other.d

        def __iter__(self):
            yield from (self.x,self.d)

class Dijkstra_Heap:
    def __init__(self):
        self.heap=[]
        self.used=set()

    def __str__(self):
        return str(self.heap)

    def __repr__(self):
        return repr(self.heap)

    def __bool__(self):
        return bool(self.heap)

    def push(self,point,dist):
        heappush(self.heap,Heap_Point(point,dist))

    def pop(self):
        assert self.heap
        while True:
            x,d=heappop(self.heap)
            if x not in self.used:
                self.used.add(x)
                return x,d
        return None

    def refresh(self):
        self.used=set()

def main():
    import sys
    from heapq import heappop,heappush
    input=sys.stdin.readline

    N,M=map(int,input().split())
    inf=float("inf")

    F=[[1]*(N+1) for _ in range(N+1)]
    for _ in range(M):
        h,w,c=map(int,input().split())
        F[h][w]+=c

    V=[(1,0),(-1,0),(0,1),(0,-1)]

    Dist0=[[inf]*(N+1) for __ in range(N+1)]
    Dist1=[[inf]*(N+1) for __ in range(N+1)]
    Dist0[1][1]=0

    H=Dijkstra_Heap()
    H.push((1,1,0),0)

    while H:
        v,d=H.pop()

        i,j,m=v
        if (m==0 and d<Dist0[i][j]) or (m==1 and d<Dist1[i][j]):
            continue

        if i==j==N:
            break

        for (a,b) in V:
            if 1<=i+a<=N and 1<=j+b<=N:
                c=d+F[i+a][j+b]
                if m==0 and c<Dist0[i+a][j+b]:
                    Dist0[i+a][j+b]=c
                    H.push((i+a,j+b,0),c)

                if m==1 and c<Dist1[i+a][j+b]:
                    Dist1[i+a][j+b]=c
                    H.push((i+a,j+b,1),c)

                if m==0 and d+1<Dist1[i+a][j+b]:
                    Dist1[i+a][j+b]=d+1
                    H.push((i+a,j+b,1),d+1)

    print(min(Dist0[N][N],Dist1[N][N]))

if __name__=="__main__":
    main()