#include <bits/stdc++.h>
using namespace std;
#define rep(i,a,b) for (int i = (a); i <= (b); ++ i)
#define rrp(i,a,b) for (int i = (a); i >= (b); -- i)
#define gc() getchar()
#define fir first
#define sec second
typedef pair<int,int> pii;
typedef double db;
typedef long long ll;
template <typename tp>
inline void read(tp& x) {
  x = 0; char tmp; bool key = 0;
  for (tmp = gc(); !isdigit(tmp); tmp = gc())
    key = (tmp == '-');
  for (; isdigit(tmp); tmp = gc()) 
    x = (x << 3) + (x << 1) + (tmp ^ '0');
  if (key) x = -x;
}
template <typename tp>
inline void ckmn(tp& x,tp y) {
  x = x < y ? x : y;
}
template <typename tp>
inline void ckmx(tp& x,tp y) {
  x = x < y ? y : x;
}

const int MOD = 998244353;
inline void Add(int& x,int y) {
  x = x + y >= MOD ? x + y - MOD : x + y;
}
inline void Sub(int& x,int y) {
  x = x - y < 0 ? x - y + MOD : x - y;
}
int power(int a,int b) {
  int ret = 1;
  while (b) {
    if (b&1) ret = 1ll * ret * a % MOD;
    a = 1ll * a * a % MOD;
    b >>= 1;
  }
  return ret;
}

const int N = 410;
int n, a[N], b[N], dp[2][N][N], sz = 400;
int ijc[N], jc[N];
void prework() {
  jc[0] = 1;
  rep (i, 1, sz) jc[i] = 1ll * jc[i-1] * i % MOD;
  ijc[sz] = power(jc[sz], MOD - 2);
  rrp (i, sz-1, 0) ijc[i] = 1ll * ijc[i+1] * (i+1) % MOD;
}
int main() {
  read(n);
  rep (i, 1, n)
    read(a[i]), read(b[i]);
  prework();
  int p = 0;
  dp[0][0][0] = MOD - 1;
  int sum = 0;
  rep (i, 1, n) sum += a[i];
  rep (i, 1, n) {
    p ^= 1;
    memset(dp[p], 0, sizeof dp[p]);
    int val = 1;
    int rec = 1ll * a[i] * power(sum, MOD - 2) % MOD;
    rep (j, 0, b[i]-1) {
      rep (ta, 0, sz - j) rep (tb, 0, sz) {
	Sub(dp[p][ta + j][tb], 1ll * dp[p^1][ta][tb] * ijc[j] % MOD * val % MOD);
      }
      val = 1ll * val * rec % MOD;
    }
    rep (ta, 0, sz) rep (tb, 0, sz - a[i])
      Add(dp[p][ta][tb + a[i]], dp[p^1][ta][tb]);
  }
  int ans = 0;
  rep (ta, 0, sz) rep (tb, 0, sz) if (dp[p][ta][tb] && tb < sum) {
    int pro = 1ll * tb * power(sum, MOD - 2) % MOD;
    int val = dp[p][ta][tb];
    // \sum_{n >= ta} n^{d ta} * val * pro^n / {pro^ta}
    // ta! * 1 / ((1 - pro * x)^{ta+1})
    int tmp = power(1 - pro + MOD, ta + 1);
    tmp = power(tmp, MOD - 2);
    tmp = 1ll * jc[ta] * tmp % MOD;
    tmp = 1ll * tmp * val % MOD;
    // pro = power(pro, ta);
    // tmp = 1ll * tmp * power(pro, MOD - 2) % MOD;
    Add(ans, tmp);
  }
  printf("%d\n", ans);
  return 0;
}
