let INF = 0x3f3f3f3f;
let N = 100010;
let idx = 0;
let head = new Int32Array(N).fill(-1);
let next = [];
let edge = [];
let w = [];

let dist = new Int32Array(N).fill(INF);
let queue = [];
let st = new Int32Array(N);

let insert = (a, b, c) => {
    edge[idx] = b;
    w[idx] = c;
    next[idx] = head[a];
    head[a] = idx++;
}

let spfa = () => {
    dist[1] = 0;
    queue.push(1);
    st[1] = true;
    while (queue.length) {
        let top = queue.shift();
        st[top] = false;
        for (let i = head[top]; i !== -1; i = next[i]) {
            let point = edge[i];
            if (dist[point] > dist[top] + w[i]) {
                dist[point] = dist[top] + w[i];
                if (!st[point]) {
                    queue.push(point);
                    st[point] = true;
                }
            }
        }
    }
}

let n = 0,
    m = 0;

let buf = '';
process.stdin.on('readable', function () {
    let chunk = process.stdin.read();
    if (chunk) buf += chunk.toString();
});
let getInputNums = line => line.split(' ').filter(s => s !== '').map(x => parseInt(x));
let getInputStr = line => line.split(' ').filter(s => s !== '');
process.stdin.on('end', function () {
    buf.split('\n').forEach(function (line, lineIdx) {
        if (lineIdx === 0) {
            n = getInputNums(line)[0];
            m = getInputNums(line)[1];
        } else if (lineIdx <= m) {
            let arr = getInputNums(line);
            let a = arr[0];
            let b = arr[1];
            let c = arr[2];
            insert(a, b, c);
            if (lineIdx === m) {
                spfa();
                if (dist[n] === INF) console.log('impossible');
                else console.log(dist[n]);

            }
        }
    });
});