Problem:
Score : 100 points

Problem Statement

There are N children, numbered 1, 2, \ldots, N.

They have decided to share K candies among themselves.
Here, for each i (1 \leq i \leq N), Child i must receive between 0 and a_i candies (inclusive).
Also, no candies should be left over.

Find the number of ways for them to share candies, modulo 10^9 + 7.
Here, two ways are said to be different when there exists a child who receives a different number of candies.

Constraints

All values in input are integers.

1 \leq N \leq 100

0 \leq K \leq 10^5

0 \leq a_i \leq K

Input

Input is given from Standard Input in the following format:

N K
a_1 a_2 \ldots a_N

Output

Print the number of ways for the children to share candies, modulo 10^9 + 7.

Sample Input 1

3 4
1 2 3

Sample Output 1

5

There are five ways for the children to share candies, as follows:

(0, 1, 3)

(0, 2, 2)

(1, 0, 3)

(1, 1, 2)

(1, 2, 1)

Here, in each sequence, the i-th element represents the number of candies that Child i receives.

Sample Input 2

1 10
9

Sample Output 2

0

There may be no ways for the children to share candies.

Sample Input 3

2 0
0 0

Sample Output 3

1

There is one way for the children to share candies, as follows:

(0, 0)

Sample Input 4

4 100000
100000 100000 100000 100000

Sample Output 4

665683269

Be sure to print the answer modulo 10^9 + 7.

Solution in C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include<bits/stdc++.h>
using namespace std;
#define rep(i,x,y) for(int i=x;i<y;i++)
#define print(A,x,n) rep(I,0,n){cout<<(I ? " ":"")<<A[I]x;}cout<<endl;
#define pprint(A,y,m,n) rep(J,0,m){print(A[J],y,n);}
const long mod=1e9+7;
const int size=1e5;
const int inf=1e9;
int main(){
	int N,K; cin>>N>>K;
	int A[N];
	rep(i,0,N) cin>>A[i];
	long dp[N+1][K+1] = {}; dp[0][0] = 1;
	rep(i,0,N){
		long res = 0;
		rep(j,0,A[i]){
			(res += dp[i][j]) %= mod;
			dp[i+1][j] = res;
		}
		rep(j,A[i],K+1){
			(res += dp[i][j]) %= mod;
			dp[i+1][j] = res;
            (res += mod - dp[i][j-A[i]]) %= mod;
		}
	}cout<<dp[N][K]<<endl;
}