Answer based on context:

Richard arrived at Acre on 8 June 1191 and immediately began supervising the construction of siege weapons to assault the city, which was captured on 12 July. Richard, Philip, and Leopold quarrelled over the spoils of the victory. Richard cast down the German standard from the city, slighting Leopold. In the struggle for the kingship of Jerusalem, Richard supported Guy, while Philip and Leopold supported Conrad, who was related to them both. It was decided that Guy would continue to rule but that Conrad would receive the crown upon his death. Frustrated with Richard , Philip and Leopold took their armies and left the Holy Land in August. Philip left 7,000 French crusaders and 5,000 silver marks to pay them. On 18 June 1191, soon after Richard's arrival at Acre, he sent a messenger to Saladin requesting a face to face meeting. Saladin refused, saying that it was customary for kings to meet each other only after a peace treaty had been agreed, and thereafter "it is not seemly for them to make war upon each other". The two therefore never met, although they did exchange gifts and Richard had a number of meetings with Al-Adil, Saladin's brother. Saladin tried to negotiate with Richard for the release of the captured Muslim soldier garrison, which included their women and children. On 20 August, however, Richard thought Saladin had delayed too much and had 2,700 of the Muslim prisoners decapitated in full view of Saladin's army, which tried unsuccessfully to rescue them. Saladin responded by killing all of the Christian prisoners he had captured.

How many months after his arrival at Acre did Richard decapitate the Muslim prisoners?
2