import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.complex, std.container, std.math, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }

struct ModInt(long M_) {
  alias M = M_;
  int x;
  this(ModInt a) { x = a.x; }
  this(long a) { x = cast(int)(a % M); if (x < 0) x += M; }
  ref ModInt opAssign(long a) { return (this = ModInt(a)); }
  ref ModInt opOpAssign(string op)(ModInt a) {
    static if (op == "+") { x += a.x; if (x >= M) x -= M; }
    else static if (op == "-") { x -= a.x; if (x < 0) x += M; }
    else static if (op == "*") { x = cast(int)((cast(long)(x) * a.x) % M); }
    else static assert(false);
    return this;
  }
  ref ModInt opOpAssign(string op)(long a) {
    static if (op == "^^") {
      ModInt t2 = this, te = ModInt(1);
      for (long e = a; e; e >>= 1) {
        if (e & 1) te *= t2;
        t2 *= t2;
      }
      x = cast(int)(te.x);
      return this;
    } else return mixin("this " ~ op ~ "= ModInt(a)");
  }
  ModInt opUnary(string op)() const if (op == "-") { return ModInt(-x); }
  ModInt opBinary(string op, T)(T a) const { return mixin("ModInt(this) " ~ op ~ "= a"); }
  ModInt opBinaryRight(string op)(long a) const { return mixin("ModInt(a) " ~ op ~ "= this"); }
  string toString() const { return x.to!string; }
}

enum MO = 998244353;
alias Mint = ModInt!MO;

enum LIM = 410 * 410 + 10;
Mint[] inv, fac, invFac;
void prepare() {
  inv = new Mint[LIM];
  fac = new Mint[LIM];
  invFac = new Mint[LIM];
  inv[1] = 1;
  foreach (i; 2 .. LIM) {
    inv[i] = -(Mint.M / i) * inv[cast(size_t)(Mint.M % i)];
  }
  fac[0] = invFac[0] = 1;
  foreach (i; 1 .. LIM) {
    fac[i] = fac[i - 1] * i;
    invFac[i] = invFac[i - 1] * inv[i];
  }
}
Mint binom(long n, long k) {
  if (0 <= k && k <= n) {
    assert(n < LIM);
    return fac[cast(size_t)(n)] * invFac[cast(size_t)(k)] * invFac[cast(size_t)(n - k)];
  } else {
    return Mint(0);
  }
}

/*
  \prod_i (\sum_{j>=B[i]} (A[i]/S)^j (1/j!) q^j
  = \prod_i exp((A[i]/S) q) - (\sum_{0<=j<B[i]} (A[i]/S)^j (1/j!) q^j
  
  (DP)
  ... = \sum_{a,b} dp[a][b] exp((a/S) q) q^b
  
  \sum_n n (n! [q^n] exp((a/S) q) q^b - (n-1)! [q^(n-1)] exp((a/S) q) q^b)
  = \sum_n n (n! (a/S)^(n-b)/(n-b)! - (n-1)! (a/S)^(n-1-b)/(n-1-b)!)
  ...
  = -b! / (1-p)^(b+1)
*/

void main() {
  prepare();
  
  try {
    for (; ; ) {
      const N = readInt();
      auto A = new int[N];
      auto B = new int[N];
      foreach (i; 0 .. N) {
        A[i] = readInt();
        B[i] = readInt();
      }
      
      const ASum = A.sum;
      const BSum = B.sum;
      auto crt = new Mint[][](ASum + 1, BSum + 1);
      crt[0][0] = 1;
      foreach (i; 0 .. N) {
        auto nxt = new Mint[][](ASum + 1, BSum + 1);
        foreach (a; 0 .. ASum + 1) foreach (b; 0 .. BSum + 1) {
          if (crt[a][b].x) {
            // exp
            nxt[a + A[i]][b] += crt[a][b];
            // poly
            Mint pp = 1;
            foreach (j; 0 .. B[i]) {
              nxt[a][b + j] -= crt[a][b] * pp * invFac[j];
              pp *= Mint(A[i]) * inv[ASum];
            }
          }
        }
        crt = nxt;
      }
      debug {
        if (ASum <= 6) {
          foreach (a; 0 .. ASum + 1) {
            writefln("crt[%s] = %s", a, crt[a]);
          }
        }
      }
      
      Mint ans;
      foreach (a; 0 .. ASum) {
        // (1 - a/S)^-(b+1) = (S / (S - a))^(b+1)
        Mint qq = 1;
        foreach (b; 0 .. BSum + 1) {
          qq *= ASum * inv[ASum - a];
          if (crt[a][b].x) {
            ans += crt[a][b] * -fac[b] * qq;
          }
        }
      }
      writeln(ans);
    }
  } catch (EOFException e) {
  }
}


Measured memory_bytes: 12668