#include <bits/stdc++.h>
using namespace std;
template <typename T>
inline bool upmin(T &x, T y) {
  return y <= x ? x = y, 1 : 0;
}
template <typename T>
inline bool upmax(T &x, T y) {
  return x <= y ? x = y, 1 : 0;
}
const long double eps = 1e-9;
const long double pi = acos(-1);
const int oo = 1 << 30;
const long long loo = 1ll << 60;
const int MAXN = 400005;
const int MAXM = 4005;
const int MAXP = 7000000;
const int MX = 20;
const int G = 3;
const int mods = 1e9 + 7;
const int Gi = (mods + 1) / G;
const int SZ = 10;
const int inv2 = (mods + 1) >> 1;
const int INF = 0x3f3f3f3f;
namespace FastIO {
constexpr int SIZE = (1 << 21) + 1;
int num = 0, f;
char ibuf[SIZE], obuf[SIZE], que[65], *iS, *iT, *oS = obuf,
                                                *oT = obuf + SIZE - 1, c;
inline void flush() {
  fwrite(obuf, 1, oS - obuf, stdout);
  oS = obuf;
}
inline void putc(char c) {
  *oS++ = c;
  if (oS == oT) flush();
}
inline void getc(char &c) {
  for (c = (iS == iT ? (iT = ((iS = ibuf) + fread(ibuf, 1, SIZE, stdin)),
                       (iS == iT ? EOF : *iS++))
                     : *iS++);
       (c == ' ' || c == '\0' || c == '\n') && c != EOF;
       c = (iS == iT ? (iT = ((iS = ibuf) + fread(ibuf, 1, SIZE, stdin)),
                       (iS == iT ? EOF : *iS++))
                     : *iS++))
    ;
}
inline void reads(char *st) {
  char c;
  int n = 0;
  getc(st[++n]);
  for (c = (iS == iT ? (iT = ((iS = ibuf) + fread(ibuf, 1, SIZE, stdin)),
                       (iS == iT ? EOF : *iS++))
                     : *iS++);
       c != ' ' && c != '\0' && c != '\n';
       c = (iS == iT ? (iT = ((iS = ibuf) + fread(ibuf, 1, SIZE, stdin)),
                       (iS == iT ? EOF : *iS++))
                     : *iS++))
    st[++n] = c;
  st[++n] = '\0';
}
template <class I>
inline void read(I &x) {
  for (f = 1, c = (iS == iT ? (iT = ((iS = ibuf) + fread(ibuf, 1, SIZE, stdin)),
                              (iS == iT ? EOF : *iS++))
                            : *iS++);
       c < '0' || c > '9';
       c = (iS == iT ? (iT = ((iS = ibuf) + fread(ibuf, 1, SIZE, stdin)),
                       (iS == iT ? EOF : *iS++))
                     : *iS++))
    if (c == '-') f = -1;
  for (x = 0; c >= '0' && c <= '9';
       c = (iS == iT ? (iT = ((iS = ibuf) + fread(ibuf, 1, SIZE, stdin)),
                       (iS == iT ? EOF : *iS++))
                     : *iS++))
    x = (x << 3) + (x << 1) + (c & 15);
  x *= f;
}
template <class I>
inline void print(I x) {
  if (x < 0) putc('-'), x = -x;
  if (!x) putc('0');
  while (x) que[++num] = x % 10 + 48, x /= 10;
  while (num) putc(que[num--]);
}
inline void putstr(string st) {
  for (int i = 0; i < (int)st.size(); ++i) putc(st[i]);
}
struct Flusher_ {
  ~Flusher_() { flush(); }
} io_Flusher_;
}  // namespace FastIO
using FastIO ::getc;
using FastIO ::print;
using FastIO ::putc;
using FastIO ::putstr;
using FastIO ::read;
using FastIO ::reads;
priority_queue<pair<long long, long long> > heap;
vector<int> V;
long long disS[MAXN], disT[MAXN], Ans[MAXN];
int n, m, Case, edgenum = 0, len = 0, vis[MAXN], f[MAXN], head[MAXN], wh[MAXN],
                L[MAXN], R[MAXN], stk[MAXN], stke[MAXN], top = 0, tope = 0;
struct enode {
  int frm, to, c, nxt;
} e[MAXN << 1];
struct Enode {
  int u, v, id;
};
vector<Enode> E1, E2;
vector<pair<long long, long long> > e1[MAXN], e2[MAXN];
long long dist[MAXN];
void add(int u, int v, int c, int id) {
  e[++edgenum] = (enode){u, v, c, head[u]}, head[u] = edgenum;
  e[++edgenum] = (enode){v, u, c, head[v]}, head[v] = edgenum;
}
int find(int x) { return f[x] == x ? f[x] : f[x] = find(f[x]); }
void Init(vector<Enode> &E, long long *dist, int S) {
  for (int i = 1; i <= n; ++i) dist[i] = loo, vis[i] = 0;
  dist[S] = 0, heap.push(make_pair(0, S));
  while (!heap.empty()) {
    int q = heap.top().second;
    heap.pop();
    if (vis[q]) continue;
    vis[q] = 1;
    for (int i = head[q]; i; i = e[i].nxt) {
      int v = e[i].to, c = e[i].c;
      if (dist[v] > dist[q] + c)
        dist[v] = dist[q] + c, heap.push(make_pair(-dist[v], v));
    }
  }
  for (int i = 1; i <= n; ++i) f[i] = i;
  for (int i = 1; i <= edgenum; ++i) {
    int u = e[i].frm, v = e[i].to, c = e[i].c;
    if (dist[v] == dist[u] + c && find(u) != find(v))
      f[find(u)] = find(v), E.push_back(((Enode){u, v, i}));
  }
}
void dfs(int x) {
  stk[++top] = x;
  if (x == n) {
    len = tope;
    for (int i = 1; i <= top; ++i) V.push_back(stk[i]);
    for (int i = 1; i <= tope; ++i) wh[stke[i]] = i;
    return;
  }
  for (auto v : e1[x]) stke[++tope] = v.second, dfs(v.first), --tope;
  --top;
}
void Dfs1(int x, int nw) {
  if (nw < (int)V.size() && V[nw] == x) ++nw;
  L[x] = nw;
  for (auto v : e1[x]) Dfs1(v.first, nw);
}
void Dfs2(int x, int nw) {
  if (nw < (int)V.size() && V[nw] == x) ++nw;
  R[x] = nw;
  for (auto v : e2[x]) Dfs2(v.first, nw);
}
long long tag[MAXN << 2];
void build(int x, int l, int r) {
  tag[x] = loo;
  if (l == r) return;
  int mid = (l + r) >> 1;
  build(x << 1, l, mid);
  build(x << 1 | 1, mid + 1, r);
}
void update(int x, int l, int r, int L, int R, long long y) {
  if (l >= L && r <= R) {
    upmin(tag[x], y);
    return;
  }
  int mid = (l + r) >> 1;
  if (R <= mid)
    update(x << 1, l, mid, L, R, y);
  else if (L > mid)
    update(x << 1 | 1, mid + 1, r, L, R, y);
  else
    update(x << 1, l, mid, L, mid, y),
        update(x << 1 | 1, mid + 1, r, mid + 1, R, y);
}
void solve(int x, int l, int r, long long y) {
  upmin(y, tag[x]);
  if (l == r) {
    Ans[l] = y;
    return;
  }
  int mid = (l + r) >> 1;
  solve(x << 1, l, mid, y);
  solve(x << 1 | 1, mid + 1, r, y);
}
signed main() {
  read(n), read(m), read(Case);
  for (int i = 1, u, v, c; i <= m; ++i)
    read(u), read(v), read(c), add(u, v, c, i);
  Init(E1, disS, 1);
  Init(E2, disT, n);
  for (auto v : E1) e1[v.u].push_back(make_pair(v.v, v.id));
  for (auto v : E2) e2[v.u].push_back(make_pair(v.v, v.id));
  dfs(1), Dfs1(1, 0), reverse(V.begin(), V.end()), Dfs2(n, 0);
  build(1, 1, len);
  for (int i = 1; i <= edgenum; ++i) {
    int u = e[i].frm, v = e[i].to, c = e[i].c;
    if (wh[i]) continue;
    if (L[u] <= len - R[v] + 1)
      update(1, 1, len, L[u], len - R[v] + 1, disS[u] + disT[v] + c);
  }
  solve(1, 1, len, loo);
  while (Case--) {
    int x, y;
    read(x), read(y);
    if (!wh[x * 2 - 1] && !wh[x * 2]) {
      long long X = disS[e[x * 2 - 1].frm] + y + disT[e[x * 2 - 1].to];
      long long Y = disS[e[x * 2].frm] + y + disT[e[x * 2].to];
      print(min(disS[n], min(X, Y)));
    } else {
      int id = (wh[x * 2 - 1] ? x * 2 - 1 : x * 2);
      if (e[id].c >= y)
        print(disS[n] - e[id].c + y);
      else
        print(min(disS[n] - e[id].c + y, Ans[wh[id]]));
    }
    putc('\n');
  }
  return 0;
}
