#include <bits/stdc++.h>
using namespace std;
#define int long long
#define endl "\n"
const int INF = 0x3f3f3f3f;
const int N = 110;

int mp[N][N], dis[N][N];
int n, m, a, b, c, ans;

void floyd() {
    for (int k = 1; k <= n; k++) {
        for (int i = 1; i < k; i++)         // 枚举ij
            for (int j = i + 1; j < k; j++) // 注意ijk不能相同
                if (ans > mp[i][j] + dis[j][k] + dis[k][i])
                    ans = mp[i][j] + dis[j][k] + dis[k][i];

        for (int i = 1; i <= n; i++) // 原floyd
            for (int j = 1; j <= n; j++)
                if (mp[i][j] > mp[i][k] + mp[k][j])
                    mp[i][j] = mp[i][k] + mp[k][j];
    }
}
signed main() {
    while (cin >> n >> m && (~n && ~m)) {
        for (int i = 1; i <= n; i++)
            for (int j = 1; j <= n; j++)
                if (i == j)
                    mp[i][j] = dis[i][j] = 0;
                else
                    mp[i][j] = dis[i][j] = INF;
        while (m--) {
            cin >> a >> b >> c;
            if (c < mp[a][b]) // 防重边，怕了怕了
                mp[a][b] = dis[a][b] = c;
            if (c < mp[b][a])
                mp[b][a] = dis[b][a] = c;
        }
        ans = INF;
        floyd();
        if (ans == INF)
            puts("It's impossible.");
        else
            printf("%lld\n", ans);
    }
}