from heapq import heappop, heappush

INF = 10 ** 15

N, M = map(int, input().split())

c = [[0] * N for _ in range(N)]
for _ in range(M):
    h, w, cc = map(int, input().split())
    c[h - 1][w - 1] = cc

dp0 = [[INF] * N for _ in range(N)]
dp1 = [[INF] * N for _ in range(N)]
dp0[0][0] = 0
q = [(0, 0, 0, 0)]
while q:
    cc, h, w, t = heappop(q)
    if t == 0:
        if dp0[h][w] < cc:
            continue
        if h != 0:
            nh = h - 1
            nw = w
            if dp0[nh][nw] > dp0[h][w] + 1 + c[nh][nw]:
                dp0[nh][nw] = dp0[h][w] + 1 + c[nh][nw]
                heappush(q, (dp0[nh][nw], nh, nw, 0))
            if c[nh][nw] > 0 and dp1[nh][nw] > dp0[h][w] + 1:
                dp1[nh][nw] = dp0[h][w] + 1
                heappush(q, (dp1[nh][nw], nh, nw, 1))
        if h != N - 1:
            nh = h + 1
            nw = w
            if dp0[nh][nw] > dp0[h][w] + 1 + c[nh][nw]:
                dp0[nh][nw] = dp0[h][w] + 1 + c[nh][nw]
                heappush(q, (dp0[nh][nw], nh, nw, 0))
            if c[nh][nw] > 0 and dp1[nh][nw] > dp0[h][w] + 1:
                dp1[nh][nw] = dp0[h][w] + 1
                heappush(q, (dp1[nh][nw], nh, nw, 1))
        if w != 0:
            nh = h
            nw = w - 1
            if dp0[nh][nw] > dp0[h][w] + 1 + c[nh][nw]:
                dp0[nh][nw] = dp0[h][w] + 1 + c[nh][nw]
                heappush(q, (dp0[nh][nw], nh, nw, 0))
            if c[nh][nw] > 0 and dp1[nh][nw] > dp0[h][w] + 1:
                dp1[nh][nw] = dp0[h][w] + 1
                heappush(q, (dp1[nh][nw], nh, nw, 1))
        if w != N - 1:
            nh = h
            nw = w + 1
            if dp0[nh][nw] > dp0[h][w] + 1 + c[nh][nw]:
                dp0[nh][nw] = dp0[h][w] + 1 + c[nh][nw]
                heappush(q, (dp0[nh][nw], nh, nw, 0))
            if c[nh][nw] > 0 and dp1[nh][nw] > dp0[h][w] + 1:
                dp1[nh][nw] = dp0[h][w] + 1
                heappush(q, (dp1[nh][nw], nh, nw, 1))
    elif t == 1:
        if dp1[h][w] < cc:
            continue
        if h != 0:
            nh = h - 1
            nw = w
            if dp1[nh][nw] > dp1[h][w] + 1 + c[nh][nw]:
                dp1[nh][nw] = dp1[h][w] + 1 + c[nh][nw]
                heappush(q, (dp1[nh][nw], nh, nw, 1))
        if h != N - 1:
            nh = h + 1
            nw = w
            if dp1[nh][nw] > dp1[h][w] + 1 + c[nh][nw]:
                dp1[nh][nw] = dp1[h][w] + 1 + c[nh][nw]
                heappush(q, (dp1[nh][nw], nh, nw, 1))
        if w != 0:
            nh = h
            nw = w - 1
            if dp1[nh][nw] > dp1[h][w] + 1 + c[nh][nw]:
                dp1[nh][nw] = dp1[h][w] + 1 + c[nh][nw]
                heappush(q, (dp1[nh][nw], nh, nw, 1))
        if w != N - 1:
            nh = h
            nw = w + 1
            if dp1[nh][nw] > dp1[h][w] + 1 + c[nh][nw]:
                dp1[nh][nw] = dp1[h][w] + 1 + c[nh][nw]
                heappush(q, (dp1[nh][nw], nh, nw, 1))
print(min(dp0[N - 1][N - 1], dp1[N - 1][N - 1]))