#include <bits/stdc++.h> using namespace std; typedef long long lint; typedef long double llf; typedef pair<int, int> pi; const int mod = 1e9 + 7; lint ipow(lint x, lint p){ lint ret = 1, piv = x % mod; while(p){ if(p&1) ret *= piv; piv *= piv; ret %= mod; piv %= mod; p >>= 1; } return ret % mod; } int n; lint p[5005], d[5005], fact[5005]; lint dp[2][5005][5005]; int main(){ cin >> n; lint ans = 0; fact[0] = 1; for(int i=1; i<=n; i++){ fact[i] = fact[i-1] * i % mod; cin >> p[i] >> d[i]; p[i] *= ipow(1000000000, mod - 2); p[i] %= mod; } dp[0][0][0] = 1; for(int i=1; i<=n; i++){ for(int j=0; j<=n; j++){ dp[0][i][j] = dp[0][i-1][j]; if(j > 0) dp[0][i][j] += dp[0][i-1][j-1] * (mod + 1 - p[i]); dp[0][i][j] %= mod; dp[1][i][j] = dp[1][i-1][j] + dp[0][i-1][j] * (p[i] * d[i] % mod); if(j > 0) dp[1][i][j] += dp[1][i-1][j-1] * (mod + 1 - p[i]); dp[1][i][j] %= mod; } } lint ret = 0; for(int i=0; i<n; i++){ ret += dp[1][n][i] * (fact[i] * fact[n-1-i] % mod) % mod; ret %= mod; } ret *= ipow(fact[n], mod - 2); ret %= mod; cout << ret; }

S.cpp: In function 'int main()':
S.cpp:26:7: warning: unused variable 'ans' [-Wunused-variable]
  lint ans = 0;
       ^