proof of summation by induction

Question

Let m and n be positive integers. Let f(i,j) be an expression defined for every i∈{1,…,m} and every j∈{1,…,n}. Prove that ∑i=1m∑j=1nf(i,j)=∑j=1n∑i=1mf(i,j).

(Hint: Use induction on n.)