#include <bits/stdc++.h>
using namespace std;
#define int long long 
const int N = 1e5+10;
const int mod = 100003;
int n,m;
int qmi(int a,int k){
    int res = 1;
    while(k){
        if(k&1) res = res * a % mod;
        a = a * a % mod;
        k >>= 1;
    }
    return res;
}
int a[N],b[N];
int x[N];
double dp[N][3];
void solve(){
    cin >> n;
    for(int i = 1;i<=n;i++) cin >> x[i];
    // memset(dp,0x3f,);
    for(int i = 0;i<=n;i++){
        for(int j = 0;j<=1;j++){
            dp[i][j] = 2e9;
        }
    }
    for(int i = 1;i<n;i++){
        cin >> a[i] >> b[i + 1];
    }
    dp[1][0] = x[1];
    for(int i = 2;i<=n;i++){
        dp[i][0] = min({dp[i][0],dp[i - 1][0] + x[i] - x[i - 1],dp[i - 1][1] + x[i] - x[i - 1] + b[i - 1]/1.3});
        dp[i][1] = min({dp[i][1],dp[i - 1][0] + b[i] / 0.7 + x[i] - x[i - 1],dp[i - 1][0] + a[i - 1] / 0.7});
        if(b[i-1]>a[i - 1]){
            dp[i][1] = min({dp[i][1],dp[i - 1][1]+(b[i - 1] - a[i - 1])/1.3});
        }else{
            dp[i][1] = min({dp[i][1],dp[i - 1][1] + (a[i - 1] - b[i - 1]) / 0.7});
        }
    }
    printf("%.2lf",min(dp[n][0],dp[n][1]+b[n]*1.0/1.3));
//  cout << min(dp[n][0],dp[n][1]+b[n]*1.0/1.3) << '\n';
}
signed main(){
    int _ = 1;
    // cin >> _;
    while(_--){
        solve();
    }
    return 0;
}