An exponential improvement for diagonal Ramsey

The Ramsey number $R(k)$ is the minimum $n\in \mathbb{N}$ such that every red-blue colouring of the edges of the complete graph $K_n$ on $n$ vertices contains a monochromatic copy of $K_k$. We prove that $$R(k)⩽(4-\epsilon {)}^k$$ for some constant $\epsilon >0$. This is the first exponential improvement over the upper bound of Erdős and Szekeres, proved in 1935.