// Written by newbiechd
#include <cstdio>
#include <cctype>
#include <cstring>
using namespace std;

const int BUF = 100003;
inline char getChar() {
  static char buf[BUF], *bufS, *bufT;
  if (bufS == bufT) {
    bufT = (bufS = buf) + fread(buf, 1, BUF, stdin);
    if (bufS == bufT) return EOF;
  }
  return *bufS++;
}
inline int read() {
  int f = 0;
  char ch;
  while (!isdigit(ch = getChar())) {}
  do f = f * 10 + (ch ^ 48);
  while (isdigit(ch = getChar()));
  return f;
}

const int maxN = 403, mod = 998244353;
int a[maxN], b[maxN], inv[maxN], fac[maxN], bin[maxN][maxN];
int p[maxN][maxN], temp[maxN][maxN];

inline int power(int x, int y) {
  int o = 1;
  while (y) {
    if (y & 1) o = 1ll * o * x % mod;
    x = 1ll * x * x % mod, y >>= 1;
  }
  return o;
}

int main() {
  int n = read(), i, j, k, t, A = 0, B = 0, ans = 0;
  for (i = 1; i <= n; ++i)
    A += a[i] = read(), b[i] = read();
  inv[1] = 1;
  for (i = 2; i < maxN; ++i)
    inv[i] = -1ll * (mod / i) * inv[mod % i] % mod;
  fac[0] = 1;
  for (i = 1; i < maxN; ++i)
    fac[i] = 1ll * fac[i - 1] * i % mod;
  k = power(A, mod - 2);
  for (i = 1; i <= n; ++i) {
    t = 1ll * k * a[i] % mod, bin[i][0] = 1;
    for (j = 1; j < b[i]; ++j)
      bin[i][j] = 1ll * bin[i][j - 1] * t % mod * inv[j] % mod;
  }
  
  p[0][0] = 1, A = 0;
  for (i = 1; i <= n; ++i) {
    for (j = 0; j <= A; ++j)
      for (k = 0; k <= B; ++k) {
        temp[j + a[i]][k] = (temp[j + a[i]][k] + p[j][k]) % mod;
        for (t = 0; t < b[i]; ++t)
          temp[j][k + t] = (-1ll * p[j][k] * bin[i][t] + temp[j][k + t]) % mod;
      }
    A += a[i], B += b[i];
    for (j = 0; j <= A; ++j) {
      memcpy(p[j], temp[j], sizeof(int) * B);
      memset(temp[j], 0, sizeof(int) * B);
    }
  }
  
  for (i = 0; i <= A; ++i)
    for (j = 0; j <= B; ++j)
      p[i][j] = -p[i][j];
  ++p[A][0];
  k = power(A, mod - 2);
  for (i = 0; i < A; ++i) {
    t = 1ll * k * i % mod;
    for (j = 0; j <= B; ++j)
      ans = (1ll * p[i][j] * fac[j] % mod * power(1 - t, mod - j - 2) + ans)
        % mod;
  }
  printf("%d\n", (ans + mod) % mod);
  return 0;
}
