Accepted

422.cpp

@@ -10,39 +10,34 @@
 using namespace std;
 
 const int MAX = 3333;
-double p[MAX],f[MAX][MAX],t,now;
-int n;
-
-void init()
-{
-        cin >> n >> t;
-        for (int i = 1; i <= n; ++i) cin >> p[i];
-}
-
-void solve()
-{
-        for (int y = 1; y <= n; ++y) 
-                for (int x = y-1; x >= 0; --x) {
-                        now = (double)1 - p[x+1];
-                        f[x][y] = (y - x + t) + (y - x) * p[x+1];
-                        for (int z = x + 2; z <= y; ++z) {
-                                f[x][y] += now * p[z] * (y - z + 1 + f[z-1][y]);
-                                now *= ((double)1 - p[z]);
-                        }
-                        f[x][y] /= ((double)1-p[x+1]);
-                }
-        cout << setiosflags(ios::fixed)
-                << setprecision(8) 
-                << f[0][n] << endl;
-}
+double p[MAX],q[MAX],f[MAX],now,add;
+int n,t;
 
 int main()
 {
-        freopen("test.in","r",stdin);
-        ios::sync_with_stdio(false);
-
-        init();
-        solve();
-
-        return 0;
-}
+	ios::sync_with_stdio(false);
+
+	cin >> n >> t;
+	for (int i = 1; i <= n; ++i) {
+		cin >> p[i];
+		q[i] = 1 - p[i];
+	}
+	for (int i = n-1; i >= 0; --i) {
+		for (int j = 1; i + j <= n; ++j) {
+			if (j == 1) {
+				now = 1 + t + p[i+1] + q[i+1] * f[i+1];
+				f[i] = now / q[i+1];
+				add = q[i+1];
+			} else {
+				add *= q[i+j];
+				now += 2 + add * (f[i+j] - f[i+j-1] - 1);
+				f[i] = min(f[i],now / q[i+1]);
+			}
+		}
+	}
+	cout << setiosflags(ios::fixed)
+	     << setprecision(7)
+	     << f[0] << endl;
+
+	return 0;
+}