from heapq import heappush,heappop

INF = int(1e15)
dh = [1,0,-1,0]
dw = [0,1,0,-1]

N, M = map(int,input().split())
dist0 = [[INF for j in range(N)] for i in range(N)]
dist1 = [[INF for j in range(N)] for i in range(N)]
c = [[0 for j in range(N)] for i in range(N)]
dist0[0][0] = 0
dist1[0][0] = 0
for i in range(M):
    H, W, C = map(int,input().split())
    H -= 1
    W -= 1
    c[H][W] = C

q = [(0,0,0,0)]
while q:
    C, H, W, used = heappop(q)
    if(used == 0 and dist0[H][W] < C):
        continue
    if(used == 1 and dist1[H][W] < C):
        continue
    for i in range(4):
        nH = H + dh[i]
        nW = W + dw[i]
        if(nH < 0 or nH >= N):
            continue
        if(nW < 0 or nW >= N):
            continue
        nC0 = C + c[nH][nW] + 1
        nC1 = C + 1
        if(used == 0):
            if(dist0[nH][nW] > nC0):
                heappush(q,(nC0,nH,nW,0))
                dist0[nH][nW] = nC0
            if(dist1[nH][nW] > nC1):
                heappush(q,(nC1,nH,nW,1))
                dist1[nH][nW] = nC1
        else:
            if(dist1[nH][nW] > nC0):
                heappush(q,(nC0,nH,nW,1))
                dist1[nH][nW] = nC0

print(min(dist0[N-1][N-1],dist1[N-1][N-1]))