luogu 1131

#include <cstdio>
#include <cstring>
#include <algorithm>
#define MAXN 500010

using namespace std;

long long n, s, dis[MAXN], vis[MAXN], now[MAXN];
long long ans, x, y, z, cnt;

struct E {
    long long from, to, nxt, vl;
} es[MAXN];

long long read() {
    long long f = 1, k = 0 ;
    char c = getchar();
    while(c > '9' || c < '0') {
        if(c == '-') {
            f = -1;
        }
        c = getchar();
    }
    while(c <= '9' && c >= '0') {
        k = k * 10 + c - '0';
        c = getchar();
    }
    return f * k;
}

void addedge(long long a, long long b, long long c) {
    es[++cnt].from = a;
    es[cnt].to = b;
    es[cnt].vl = c;
    es[cnt].nxt = now[a];
    now[a] = cnt;
}

long long dfs(long long a) {
    long long sum = 0, maxx = 0;
    vis[a] = 1;
    for(int i = now[a]; i != 0; i = es[i].nxt) {
        if(!vis[es[i].to]) {
            sum ++;
            dis[es[i].to] = dis[a] + es[i].vl;
            dfs(es[i].to);
            if(maxx > dis[es[i].to]) {
                ans += maxx - dis[es[i].to];
            } else {
                if(sum == 1) {
                    maxx = dis[es[i].to];
                } else {
                    ans += (sum - 1) * (dis[es[i].to] - maxx);
                    maxx = dis[es[i].to];
                }
            }
        }
    }
    dis[a] = max(dis[a], maxx);
}

int main() {
    n = read();
    s = read();
    for(int i = 1; i < n; i ++) {
        x = read(), y = read(), z = read();
        addedge(x, y, z);
        addedge(y, x, z);
    }
    dis[s] = 0;
    dfs(s);
    printf("%lld", ans);
    return 0;
}