Subarray Jumps

There are $N$ stones in a pond, each having a value $A_i$ written on it. A frog is at stone $1$ and wants to reach stone $N$. The frog can jump from a stone $i$ to any stone $j (j>i)$. Let $d$ be the length of subarray (i.e. $j-i+1$), then the energy required for the jump is $(d \cdot A_i)-A_j$. Find the minimum **non-negative** amount of energy required by the frog to reach the $N$-th stone. **N